Auto-titration bi-level pressure support system and method of using same

ABSTRACT

A bi-level pressure support system and method of treating disordered breathing that optimizes the pressure delivered to the patient during inspiration and expiration to treat the disordered breathing while minimizing the delivered pressure for patient comfort. The pressure generating system generates a flow of breathing gas at an inspiratory positive airway pressure (IPAP) during inspiration and at an expiratory positive airway pressure (EPAP) during expirations. A controller monitor at least one of the following conditions: (1) snoring, (2) apneas, (3) hypopneas, or (4) a big leak in the pressure support system and adjusts the IPAP and the EPAP based on the occurrence of any one of these conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is Continuation and claims priority under 35 U.S.C.§120 from Ser. No. 11/217,964, filed Sep. 1, 2005, now U.S. Pat. No.7,938,114, which is a Continuation-In-Part and claims priority under 35U.S.C. §120 from U.S. patent application Ser. No. 10/788,507, filed Feb.27, 2004, now U.S. Pat. No. 7,827,988, which is a Continuation-In-Partand claims priority under 35 U.S.C. §120 from U.S. patent applicationSer. No. 10/268,406 filed Oct. 10, 2002, now U.S. Pat. No. 7,168,429,which claims priority under 35 U.S.C. §119(e) from U.S. provisionalpatent application No. 60/329,250 filed Oct. 12, 2001 and U.S.provisional patent application No. 60/331,838 filed Nov. 20, 2001, thecontents of each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a pressure support system and methodof treating disordered breathing, and, in particular, to bi-levelauto-titration pressure support system and to a method of automaticallytitrating a pressure support system to optimize the inspiratory andexpiratory pressure delivered to the patient to treat the disorderedbreathing while otherwise minimizing the delivered pressure for patientcomfort.

2. Description of the Related Art

It is well known that many individuals suffer from disordered breathingduring sleep. Obstructive sleep apnea (OSA), for example, is a commonexample of such disordered breathing suffered by millions of peoplethrough the world. OSA is a condition in which sleep is repeatedlyinterrupted by an inability to breathe, which occurs due to anobstruction of the airway; typically the upper airway or pharyngealarea. Obstruction of the airway is generally believed to be due, atleast in part, to a general relaxation of the muscles which stabilizethe upper airway segment, thereby allowing the tissues to collapse theairway.

Those afflicted with OSA experience sleep fragmentation and complete ornearly complete cessation of ventilation intermittently during sleepwith potentially severe degrees of oxyhemoglobin desaturation. Thesesymptoms may be translated clinically into extreme daytime sleepiness,cardiac arrhythmias, pulmonary-artery hypertension, congestive heartfailure and/or cognitive dysfunction. Other consequences of OSA includeright ventricular dysfunction, carbon dioxide retention duringwakefulness, as well as during sleep, and continuous reduced arterialoxygen tension. Sleep apnea sufferers may be at risk for excessivemortality from these factors as well as by an elevated risk foraccidents while driving and/or operating potentially dangerousequipment.

Even if a patient does not suffer from a complete obstruction of theairway, it is also known that adverse effects, such as arousals fromsleep, can occur where there is only a partial obstruction of theairway. Partial obstruction of the airway typically results in shallowbreathing referred to as a hypopnea. Other types of disordered breathinginclude upper airway resistance syndrome (UARS) and vibration of theairway, such as vibration of the pharyngeal wall, commonly referred toas snoring. It is also known that snoring can accompany closure of theairway leading to UARS, hypopnea, or apnea. Thus, snoring serves as anindicator that the patient is experiencing abnormal breathing.

It is known to treat such disordered breathing by applying a continuouspositive air pressure (CPAP) to the patient's airway. This positivepressure effectively “splints” the airway, thereby maintaining an openpassage to the lungs. It is also known to provide a positive pressuretherapy in which the pressure of gas delivered to the patient varieswith the patient's breathing cycle, or varies with the patient's effort,to increase the comfort to the patient. This pressure support techniqueis referred to a bi-level pressure support, in which the inspiratorypositive airway pressure (IPAP) is delivered to the patient is higherthan the expiratory positive airway pressure (EPAP).

It is further known to provide a positive pressure therapy in which acontinuous positive pressure is provided to the patient, and where thelevel of this pressure is automatically adjusted based on the detectedconditions of the patient, such as whether the patient is snoring orexperiencing an apnea, hypopnea or upper airway resistance. Thispressure support technique is referred to as an auto-titration type ofpressure support, because the pressure support device seeks to provide apressure to the patient that is only as high as necessary to treat thedisordered breathing.

Examples of conventional auto-titration pressure support systems aredisclosed in U.S. Pat. No. 5,245,995 to Sullivan et al.; U.S. Pat. Nos.5,259,373; 5,549,106, and 5,845,636 all to Gruenke et al.; U.S. Pat.Nos. 5,458,137 and 6,058,747 both to Axe et al.; U.S. Pat. Nos.5,704,345; 6,029,665; and 6,138,675 all to Berthon-Jones; U.S. Pat. No.5,645,053 to Remmers et al.; and U.S. Pat. Nos. 5,335,654; 5,490,502,5,535,739, and 5,803,066 all to Rapoport et al. All of theseconventional pressure support systems, with the exception of U.S. Pat.No. 5,645,053 to Remmers et al., are reactive to the patient's monitoredcondition. That is, once a condition occurs that indicates abnormalbreathing, the system alters the pressure support to treat thiscondition. The present inventors discovered, however, that thistreatment technique may not be suitable for all patients, and may causethe system to unnecessarily react to mild, temporary anomalies.

Furthermore, these auto-titration pressure support systems typicallyattempt to treat one condition of the patient, such as snoring or a flowlimitation in the patient's inspiratory waveform. It is believed thatthis micro, target treatment, approach, focusing on one or twoconditions, fails to provide an adequate treatment for a patient, which,in essence, is a very complicated system, affected by a variety ofvariables.

In addition, these conventional auto-titration systems present differentapproaches to detecting a condition of the patient. Each approachattempts to improve the ability to detect conditions of the patient thatare truly indicative of a breathing disorder. However, each approach isbelieved to be limited in its ability to monitor and treat a widepopulation of patients in a robust manner.

Finally, conventional auto-titration pressure support systems are CPAPsystems, and, as such, deliver a constant pressure that is the sameduring the patient's inspiratory and expiratory cycle. This approach,while acceptable to many patients, may not provide the optimum pressuresupport therapy or comfort for all patients.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide abi-level auto-titration pressure support system that overcomes theshortcomings of conventional auto-titration systems. This object isachieved according to one embodiment of the present invention byproviding a bi-level auto-titration pressure support system thatincludes a pressure generating system that generates a flow of breathinggas at an inspiratory positive airway pressure (IPAP) during inspirationand at an expiratory positive airway pressure (EPAP) during expiration.A patient circuit is coupled between the pressure generating system andan airway of the patient. A monitoring system is associated with thepatient circuit or the pressure generating system and measures ormonitors a parameter indicative of a pressure at the patient's airway, aflow of gas in the patient's airway, or both and to output a pressuresignal, a flow signal indicative thereof, respectively, or both. Acontroller coupled to the monitoring system and the pressure generatingsystem, is provided for controlling the pressure generating system basedon the output of the monitoring system. More specifically, thecontroller is programmed to monitor at least one of the followingconditions: (1) snoring, (2) apneas, (3) hypopneas, or (4) a big leak inthe pressure support system. The big leak is a system leak that issubstantially greater than any intentional system leaks. The controlleradjusts the IPAP and the EPAP based on the occurrence of any one ofconditions (1), (2), (3), or (4). In an exemplary embodiment, the IPAPand EPAP are adjusted together, so that the pressure support (PS) levelremains constant.

It is yet another object of the present invention to provide a method ofdelivering a pressure support treatment to a patient according to thepressure support system operating functions discussed above.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a pressure support system adapted tooperate according to the auto-titration technique of the presentinvention;

FIG. 2 is a schematic diagram of a control system for implementing theauto-titration technique of the present invention;

FIG. 3 is a pressure-flow diagram that illustrates the criteria fordetermining whether to initiate various control features of theauto-titration technique of the present invention;

FIGS. 4A-4C illustrate further exemplary waveforms that illustrate thedifference between an actual peak flow and a weighted peak flowQ_(Wpeak) used by the present invention;

FIG. 5 is a graph illustrating an exemplary inspiratory waveform forexplaining how the present invention calculates various parameters usedin performing the auto-titration functions;

FIG. 6 is an exemplary histogram of the weighted peak flows for thebreaths accumulated during the moving window time period;

FIGS. 7A-7E are flow charts illustrating the hypopnea detection processaccording to the principles of the present invention;

FIG. 8 is an exemplary embodiment of a patient flow waveform for use indescribing the gap filling process used in the apnea detection techniqueof the present invention;

FIG. 9 illustrates an exemplary patient pressure to describe theoperation of the apnea/hypopnea treatment procedure of the pressuresupport system;

FIGS. 10A and 10B are graphs illustrating the examples of the scatter ofweighted peak flows;

FIG. 11 is a chart illustrating a process by which the mean flow ismapped or normalized according to a variable breathing detection processof the present invention;

FIG. 12 is a chart illustrating the hysteresis threshold criteria fordeclaring that the patient is experiencing variable breathing;

FIG. 13 is a chart illustrating the pressure control operation of thevariable breathing control module of the present invention;

FIGS. 14A-14C illustrate exemplary patient inspiratory waveforms for usein explaining the roundness and flatness calculations of the presentinvention;

FIG. 15 illustrates an exemplary patient inspiratory waveform and a sinewave template for use in explaining the roundness and flatnesscalculations;

FIGS. 16A and 16B illustrate extreme examples of different sine wavetemplates;

FIG. 17 illustrates a normalization curve that is used to adjust theratio of the sine wave templates;

FIGS. 18A and 18B illustrate sine wave templates showing how theamplitude of the template is corrected according the roundness andflatness calculation process of the present invention;

FIGS. 19A and 19B illustrate an exemplary patient inspiratory waveformand a corresponding sine wave template for use in explaining theroundness and flatness calculations;

FIG. 20 illustrates a patient inspiratory waveform showing how flatnessis calculated according to the principles of the present invention;

FIG. 21 illustrates a patient inspiratory waveform showing how roundnessis calculated according to the principles of the present invention;

FIG. 22 illustrates a patient inspiratory waveform showing how skewnessis calculated according to the principles of the present invention;

FIG. 23 illustrates how respiratory parameter data is accumulated fortrend analysis purposes according to the principles of the presentinvention;

FIG. 24 is a chart illustrating the trend analysis technique of thepresent invention;

FIG. 25 is a chart explaining the voting process carried out during along-term trend analysis according to the present invention;

FIG. 26 illustrates an exemplary pressure curve output by the pressuresupport system during a pressure increase operation;

FIGS. 27A and 27B illustrate further exemplary pressure curves output bythe pressure support system of the present invention;

FIG. 28 illustrates an exemplary patient flow waveform during anobstructive/restrictive apnea/hypopnea event;

FIG. 29 illustrates an exemplary patient flow waveform during a centralapnea/hypopnea event;

FIG. 30 illustrates a further exemplary patient flow waveform during anobstructive/restrictive apnea/hypopnea event; and

FIGS. 31-33 are waveforms illustrating examples of how the IPAP and EPAPlevels can be controlled during a ramp cycle according to the principlesof the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

A. System Hardware

The basic components of a pressure support system 30 that is adapted toimplement the bi-level auto-titration technique according to theprinciples of the present invention is discussed below with reference toFIG. 1. Pressure support system 30 includes a pressure generatingsystem, generally indicated at 32, and a patient circuit 34, whichincludes a conduit 36 and a patient interface device 38. In theillustrated embodiment, pressure generating system 32 includes apressure generator 40 and a pressure control valve 42 as the outlet ofthe pressure generator.

Pressure generator 40 receives the breathing gas from a source ofbreathing gas, as indicated by arrow A, and outputs the breathing gas,as indicated by arrow B, to patient circuit 34 at a pressure that isgreater than atmosphere for delivery to the airway of a patient (notshown). In a preferred embodiment of the present invention, pressuregenerator 40 is a mechanical pressure generator, such as a blower,bellows or piston, that receives ambient air, for example, at an inletfrom the gas source. Pressure control valve 42 controls the pressure ofthe flow of breathing gas delivered to the patient via the patientcircuit by restricting the flow to the patient, by diverting flow frompatient circuit 34, as indicated by arrow C, or a combination thereof.

The present invention further contemplates controlling the pressure ofthe flow of breathing gas delivered to the patient by controlling theoperating speed of pressure generator 40, either alone or in combinationwith valve 42. Of course, valve 42 can be eliminated if operating speedalone is used to control the pressure of the flow of breathing gasdelivered to the patient. Those skilled in the art can appreciate thatother techniques for controlling the pressure of the flow of breathinggas delivered to the patient can be implemented in pressure supportsystem 30, either alone or in combination to those discussed above. Forexample, a flow restricting valve (not shown) can be provided upstreamof pressure generator 40 that controls the flow (arrow A) of gas topressure generator 40, and, hence, the pressure of the flow of gasoutput for delivery to the patient.

Typically, the source of breathing gas is the ambient atmosphere, whereits pressure is subsequently elevated for delivery to the patient by thepressure generating system. It is to be understood, that other sourcesof breathing gas are contemplated by the present invention, such asoxygen or an oxygen mixture from an oxygen source. It is to be furtherunderstood, that the present invention contemplates that pressurized aircan be provided to the airway of the patient directly from a tank ofpressurized air via the patient circuit without using a pressuregenerator, such as a blower, bellows or piston, to increase the pressureof the air. Of course, a pressure regulator, such as valve 42 would berequired to control the pressure of the gas delivered to the patient.The important feature with respect to the present invention is thatpressurized breathing gas is provided in the patient circuit fordelivery to the patient, not necessarily the source or manner in whichthe pressurized breathing gas is generated.

Although not shown in FIG. 1, the present invention also contemplatesproviding a secondary flow of gas, either alone or in combination withthe primary flow of gas (arrow A) from atmosphere. For example, a flowof oxygen from any suitable source can be provided upstream to pressuregenerator 40 or downstream of the pressure generator in the patientcircuit or at the patient interface device to control the fraction ofinspired oxygen delivered to the patient.

In the illustrated embodiment, conduit 36 in patient circuit 34 has oneend coupled to the output of the pressure generator 40 and another endcoupled to patient interface device 38. Conduit 36 is any tubing capableof carrying the gas flow from the pressure generator to the airway ofthe patient. Typically, a distal portion of the conduit 36 relative topressure generator 40 is flexible to allow for freedom of movement ofthe patient. It is to be understood that various components may beprovided in or coupled to patient circuit 34. For example, a bacteriafilter, pressure control valve, flow control valve, sensor, meter,pressure filter, humidifier and/or heater can be provided in or attachedto the patient circuit. Likewise, other components, such as mufflers andfilters can be provided at the inlet of pressure generator 40 and at theoutlet of valve 42.

Patient interface device 38 in patient circuit 34 is any device suitablefor communicating an end of conduit 36 with the airway of the patient.Examples of suitable patient interface devices include a nasal mask,oral mask or mouthpiece, nasal/oral mask, nasal cannula, trachea tube,intubation tube, hood or full face mask. It is to be understood thatthis list of suitable interface devices is not intended to be exclusiveor exhaustive.

In the single limb patient circuit of the present invention, exhaled gasfrom the patient typically exits the patient circuit via an exhaust vent43, as indicated by arrow D. In the illustrated embodiment, exhaust vent43 is provided on a distal portion of conduit 34. Depending on the tidalvolume of the patient and the pressure delivered by pressure supportsystem 30, a small percentage of the exhaled gas may travel back up theconduit into pressure support system 30 and may even be exhausted toatmosphere through the gas inlet of the pressure generator and/orthrough a pressure control valve 42, if such a valve is being used withthe pressure generator.

Typically, exhaust vent 43 is an orifice provided in the conduit thatcommunicates the interior of the conduit with atmosphere, with no activecontrol over the flow of gas from the system. It is to be understood,however, that a wide variety of exhaust devices and configurations arecontemplated for use with the pressure generating system of the presentinvention. For example, U.S. Pat. No. 5,685,296 to Zdrojkowski et al.discloses an exhalation device and method where the exhalation flow ratethrough the device remains substantially constant over a range ofpressures in the patient circuit. This exhalation device, which iscommonly referred to as a plateau exhalation valve or PEV, is suitablefor use with the pressure support system of the present invention.

As shown in FIG. 1, pressure support system 30 includes a monitoringsystem, generally indicated at 44, to monitor the flow and pressure ofgas delivered to the patient. In the illustrated embodiment, monitoringsystem 44 includes a flow sensor 46 that measures a rate at which thebreathing gas flows within patient circuit 34. The present inventioncontemplates that any suitable sensor, such as a conventionalpneumatach, can be used for flow sensor 46. It is to be furtherunderstood that flow sensor 46 need not be coupled directly to conduit36. On the contrary, the present invention contemplates the use of anysensor or a plurality of sensors that can quantitatively measure airflowin the patient circuit. For example, flow in the system can be measuredat the patient interface device or can be measured or estimated from themotor or piston speed or from torque used to provide the elevatedpressure by pressure generator 40. In short, the present inventioncontemplates any conventional technique for measuring the flow of gasdelivered to the patient.

Monitoring system 44 also includes a pressure sensor 48 that detects thepressure of the gas at the patient. In the illustrated embodiment,pressure sensor 48 is in fluid communication with patient interfacedevice 38 via a conduit 36. In this embodiment, the pressure at thepatient is estimated based on the known pressure drop that occurs intubing 36. It is to be understood, however, that the patient pressurecan be measured directly at patient interface device 38.

Pressure support system 30 includes a controller 50, which is preferablya microprocessor capable of implementing a stored algorithm, thatreceives the monitored variables, typically from flow sensor 46 andpressure sensor 48, and controls pressure generating system 32 based onthese signals. Of course, controller 50 includes the necessary memoryand processing capability to implement the features of the presentinvention. In a preferred embodiment of the present invention,controller 50 is an AMTEL AT91M55800 microcontroller that runs storedsoftware written in C programming language.

The present invention further contemplates that pressure support system30 includes an input/output interface 52 for communicating, information,data and/or instructions and any other communicable items, collectivelyreferred to as “data”, between a user and controller 50. Examples ofcommon input/output interfaces suitable for this purpose include akeypad and display. Other communication techniques, either hard-wired orwireless, are also contemplated by the present invention. For example,the present invention contemplates providing a smart card terminal thatenables data to be loaded into controller 50 from the smart card orloaded onto the smart card from the controller. Other exemplary,interface devices and techniques adapted for use with the pressuresupport system include, but are not limited to, an RS-232 port, CDreader/writer, DVD reader/writer, RF link, and modem (telephone, cableor other). In short, any conventional technique for providing,receiving, or exchanging data with controller 50 are contemplated by thepresent invention as input/output device 52.

Controller 50 also performs conventional leak estimation and respiratorycycle monitoring techniques. The present invention contemplates usingany conventional technique for calculating leak Q_(leak), which is theleakage of gas from the pressure support system and includes intentionalleaks from the exhaust vent and unintentional leaks from themask-patient interface, for example. The present invention alsocontemplates using any conventional technique for taking leak intoconsideration when determining the patient flow Q_(patient), which isthe flow of gas at the airway of the patient, and total flow Q_(total)which is the flow of gas typically measured by flow sensor 46. Forexample, U.S. Pat. No. 5,148,802 to Sanders et al., U.S. Pat. No.5,313,937 to Zdrojkowski et al., U.S. Pat. No. 5,433,193 to Sanders etal., U.S. Pat. No. 5,632,269 to Zdrojkowski et al., U.S. Pat. No.5,803,065 to Zdrojkowski et al., U.S. Pat. No. 6,029,664 to Zdrojkowskiet al., and U.S. Pat. No. 6,360,741 to Truschel, and pending U.S. patentapplication Ser. No. 09/586,054 to Frank et al. and Ser. No. 09/970,383to Jafari et al., the contents of each of which are incorporated byreference into the present invention, all teach techniques for detectingand estimating leak and managing the delivery of breathing gas to thepatient in the presence of leaks.

The present invention also contemplates using any conventional techniquefor detecting the inspiratory and expiratory phases of the patient'srespiratory cycle. For example, U.S. Pat. No. 5,148,802 to Sanders etal., U.S. Pat. No. 5,313,937 to Zdrojkowski et al., U.S. Pat. No.5,433,193 to Sanders et al., U.S. Pat. No. 5,632,269 to Zdrojkowski etal., U.S. Pat. No. 5,803,065 to Zdrojkowski et al., U.S. Pat. No.6,029,664 to Zdrojkowski et al., and pending U.S. patent applicationSer. No. 09/970,383 to Jafari et al., all teach techniques fordifferentiating between the inspiratory and expiratory phases of arespiratory cycle.

The present invention contemplates that controller 50 controls pressuregenerating system 32 such that a bi-level form of pressure support isdelivered to the patient. Under this form of pressure control, which isalso referred to as BiPAP®, an inspiratory positive airway pressure(IPAP) is delivered to the patient during the inspiratory phase, and alower expiratory positive airway pressure (EPAP) is delivered during theexpiratory phase. U.S. Pat. Nos. 5,148,802; 5,433,193; 5,632,269;5,803,065; 6,029,664; 6,305,374; and 6,539,940, the contents of whichare incorporated herein by reference, describe a bi-level pressuresupport technique.

In bi-level pressure support, the difference between the IPAP and theEPAP is referred to as the pressure support (PS) level. That isPS=IPAP−EPAP. The present invention adjusts the IPAP and EPAP so as tomaintain the pressure support at a fixed level or adjusts the IPAP andEPAP independently so that the PS level, both EPAP and IPAP changesduring the course of treatment.

The present invention contemplates storing, controlling, or monitoringthe IPAP and the EPAP levels. Thus, each pressure, IPAP and EPAP, can beadjusted independent or in unison, for example, to maintain a constantPS level. It should be understood that the present invention alsocontemplates storing, controlling, or monitoring either the IPAP or theEPAP level, and determining or controlling the other based on the PSlevel. In short, if the IPAP or EPAP is known or is used as the primarycontrol pressure, the other unknown IPAP or EPAP level can be determinedif the PS is also known. For example, the controller need not controlthe EPAP level, if it controls the IPAP level and adjusts the EPAP levelbased on the PS level. Remember that the PS level can be constant insome situations and can vary in others, as described herein. Conversely,the controller can control EPAP level as the primary pressure, andadjust the IPAP level to follow the EPAP level based on the PS.

In situations where the IPAP and EPAP are permitted to changeindependently, i.e., where the PS level is variable, the presentinvention contemplating applying a maximum (PS_(max)) and a minimum(PS_(min)) constraint for the PS level. In an exemplary embodiment ofthe present invention, PS_(min) is fixed at 2 cmH₂O and is fixed at alltimes. Thus, the PS level will be maintained at or above 2 cmH₂O at alltimes. Any attempt to adjust IPAP or EPAP so that the PS level is lessthan 2 cmH₂O will be ignored or overridden. It should be noted that thepresent invention also contemplates PS_(min) can be adjustable, so thatthe user can select the desired level or can be adjusted based on othercriteria to allow the PS level to decrease below the minimum only incertain situations.

In an exemplary embodiment of the present invention, PS_(max) is a useradjustable setting. However, once set, it remains fixed at all timesduring the operation of the system. Thus, the PS level will not exceedPS_(max), and any attempt to adjust IPAP or EPAP so that the PS level isgreater than PS_(max) will be ignored or overridden. It should be notedthat the present invention also contemplates PS_(max) can be fixed andnot user adjustable, or it can be adjustable based on certain criteriato allow the PS level to exceed the maximum only in certain situations.

B. Prioritized Controllers

The auto-titration technique implemented by pressure support system 30according to the principles of the present invention is based oncontroller 50 being programmed to operate in a such a manner that iteffectively functions as a set of prioritized controllers 100, with eachcontroller, or control layer in the controller hierarchy, competing forcontrol of the pressure support system, i.e., for control over thepressure delivered to the patient by the pressure generating system.

FIG. 2 schematically illustrates this prioritized control system, withthe priority of each control layer being identified by numerals (1)-(8)on the right side of the figure. The control layer at the uppermostportion of the figure, i.e., having the first (1) priority, is thehighest priority controller and takes precedence over all othercontrollers. The control layer at the lowermost portion of the figure,i.e., having an eighth (8) priority, is the lowest priority controllerthat only operates if no other controller is operating.

Controller 50 is further programmed to effectively provide a set ofdetectors or detection modules 102 and a set of monitors or monitoringmodules 104 so that an individual detection module and, if necessary, anindividual monitoring module is associated with each control layer.Detection modules 102 receive the raw inputs, such as the signal frompressure sensor 48, flow sensor 46, or both. Detection modules 102 alsoperform any necessary signal processing that may be necessary to providean input to the associated monitoring module. Monitoring modules 104determine, from the output of the associated detection module, whetherthe criteria for requesting activation of an associated control moduleare satisfied. If so, a request for control of the pressure supportsystem is initiated to a request processor 106, which determines whethercontrol should be turned over to the control module associated with themonitoring module making the request. The algorithm executed by thecontroller performs the request processing function based on thepriority of the control layer that is requesting control of the pressuresupport system.

Once a controller in a control layer is activated, it controls theoperation of the pressure support system and maintains control until thecondition that activated the controller is resolved or a higher prioritycontroller takes over. While in control, each controller treats thespecific event/condition by performing its control functions, such asadjusting the pressure output from the pressure support system via thepressure generating system. Each controller operates in a unique fashionbased on the type of event/condition being treated.

It should be understood that the present invention contemplates settinga prescribed minimum pressure P_(min) and a prescribed maximum pressureP_(max) that serve as absolute pressure boundaries that the pressuresupport system cannot exceed. Of course, some controllers may haveadditional constraints on how the pressure is adjusted.

Dashed line 108 in FIG. 2 delineates a difference between control layersthat are based on the conditions of the pressure support system andcontrol layers that are based on the monitored condition of the patient.More specifically, control layers having a priority of (1)-(3), whichare above line 108, are machine-based control layers that take controlof the operation of pressure support system 30 based only on thecondition of the pressure support system. On the other hand, controllayers having a priority of (4)-(8), which are below dashed line 108,are patient based control layers that take control of the pressuresupport system based on the monitored condition of the patient.

The control layers can be further subdivided into control layers thatoperate based on monitored pressure, flow, or both, and control layersthat operate based on the manual inputs, such as whether the patient hasturned the pressure support system on or activated a pressure ramp. Inthe presently preferred embodiment of the invention, only the first twocontroller layers, i.e., the control layers having priority of (1) and(2) are control layers that are based on the manual inputs from thepatient or user.

C. First and Second Priority Control Layers

The first priority control layer receives inputs 110 from theinput/output device 52. In this first control layer, the input is anindication, typically from an on/off switch or button, of whether thepatient has turned the unit on or off. Naturally, if the patient turnsthe pressure support off, this should override all other pressurecontrols, which is why it is given the highest priority in the hierarchyof control layers in the present invention. On/off detection layer 112determines, from the signal from the on/off switch or other similardevice, such as an auto on/off technique noted below, whether thepatient has activated or deactivated the pressure support system. Ofcourse, this decision will depend on whether the system is alreadyoperating at the time the on/off switch is activated. This indication isprovided to request processor 106, where it is deemed to have thehighest priority, and all other control operations are overridden sothat control of the pressure support system is given over to an on/offcontroller 114.

On/off controller 114 performs any functions that may be desired ornecessary in activating or deactivating the pressure support system. Forexample, when the pressure support system is deactivated, the pressuresupport system may perform such processes as storing current pressuresettings, compliance information, and other information in a memory orother storage device, in addition to turning off pressure generatingsystem 32. When the pressure support system is activated by the user,the system may perform activation processes, such as reading informationfrom memory or a smart card, retrieving the input settings from theinput devices, performing diagnostic functions, resetting lower prioritydetection, monitoring and control modules, and turning on the pressuregenerating system.

The second priority control layer also receives inputs 110 from theinput/output device 52. In this control layer, the input is anindication, typically from a ramp activation button, of whether thepatient has activated a pressure ramp operation. Ramp detection layer116 determines, from the signal from the on/off switch or other similardevice, whether the patient has activated the ramp activation button. Ifso, this ramp activation request is provided to request processor 106,where it is deemed to have the second highest priority, and all othercontrol operations, other than the on/off control, are overridden, andcontrol is given over to a ramp controller 118.

Ramp control module 118 causes the pressure support system to reduce theIPAP and EPAP levels to a lower setting, such as the system minimum, fora predetermined period of time or for a predetermined number ofbreathing cycles. The present invention also contemplates providing apressure ramp to the patient using any conventional pressure rampingtechnique, rather than merely dropping the pressure.

In short, when ramp controller 118 assumes control of the pressuresupport system, it overrides the current IPAP and EPAP levels deliveredto the patient and controls pressure generating system 32 so thatrelatively low IPAP and EPAP levels are delivered to the patient. Afterthe elapse of the ramp duration, which can be time based or event based(based on the passage of a predetermined number of breathing cycles) thepressure ramp control is released and another control layer takes overcontrol of the pressure support system. If the ramp feature includes anactual pressure ramp, the IPAP and EPAP levels are increased over aperiod of time, such as 5-45 minutes, or over a predetermined number ofbreathing cycles. In an exemplary embodiment, the IPAP and EPAP areincreased such that the pressure support (PS) remains constant.Thereafter, the pressure ramp control is released and another controllayer takes over control of the pressure support system. The goal ofthis embodiment of the present invention is to allow the patient tomanually override the pressure provided by the system so that thepressure is reduced to a relatively low level that allows the patient tofall asleep under this relatively low pressure and thereafter, receivethe therapeutically beneficial pressure.

A further embodiment of the present invention contemplates providing aramp function that allows the pressure support system to react torespiratory events even during a ramp function. An example, of this“smart ramp” is described with reference to FIGS. 31-33. In thisembodiment, the IPAP (FIG. 32), the EPAP (not shown), or both (FIG. 33)are permitted to be adjusted, for example, by other controller layers,even during the ramping process.

As shown in FIG. 31, the IPAP and the EPAP are lowered relativelyrapidly upon actuation of a ramp cycle at point 700. In the illustratedexemplary, embodiment, the IPAP and EPAP are lowered until the EPAPreaches an EPAP_(Ramp) level, which is set lower than the EPAP_(min)level. During this pressure decrease the PS level is kept constant.Thus, the pressure decrease for the IPAP and EPAP will have the sameslope or rate of decrease.

In an alternative embodiment, the EPAP value is decreased until itreaches EPAP_(Ramp) and the IPAP value is decreased until it reaches avalue that corresponds to EPAP_(Ramp)+PS_(min). Thus, the PS level isnot held constant so that the IPAP and EPAP values reach their minimumat the same point in time. Thus, the slope or rate of decrease for theIPAP and EPAP at the start of the ramp cycle would not be the same.

At point 702 the EPAP reaches the EPAP_(Ramp) and the pressure decreaseis terminated. The IPAP level at point 702 is at EPAP_(Ramp)+PS.Alternatively, the IPAP level is at EPAP_(Ramp)+PS_(min). In theillustrated embodiment, the IPAP and EPAP are held at this relativelylow level for a short period of time until a point 704 is reached. Atpoint 704, the pressure ramp for the IPAP and EPAP begins. The pressureincrease during the ramp cycle continues until the EPAP delivered to thepatient reaches EPAP_(min), which occurs at point 706. EPAP_(min) is theminimum EPAP level that the pressure support system will deliver to thepatient. That is, other than during a ramp cycle, the system will notallow the EPAP to fall below EPAP_(min).

In the illustrated embodiment, the pressure increase portion of the rampcycle, which begins at point 704 and ends at point 706, has a linearshape. It is to be understood, however, that the present inventioncontemplates that the pressure increase for the IPAP, EPAP or both canhave other shapes or profiles. In addition the shape for each or bothpressures can be selected by the user, as described, for example, inU.S. Pat. No. 5,682,878 to Ogden, the contents of which are incorporatedherein by reference. The present invention also contemplates that theoverall shape for the entire ramp cycle can have shapes other than thatillustrated in the figures. For example, the pressure decrease andincrease can be controlled so that the pressure waveform has a “U”shape, a parabolic shape, a skewed parabola, etc. In addition, thepresent invention contemplates beginning the pressure increase portionof the ramp cycle at point 702, i.e., the EPAP pressure reachesEPAP_(Ramp), thereby effectively eliminating the pressure hold intervalbetween points 702 and 704.

In an exemplary embodiment of the present invention, the duration of thepressure increase portion of the ramp cycle, i.e., the period of timebetween the beginning of the pressure increase (point 704) and the endof the pressure increase (point 706) is preset in the system or is setby the user. Once the duration is set or determined, the systemdetermines the appropriate slope or rate of increase so that thepressure will reach EPAP_(min) at the end of that duration and causesthe EPAP and IPAP to increase in the appropriate fashion.

The duration of the ramp can also be selected by the patient,preprogrammed into the controller, and/or can depend on whether the rampactivation device has already been activated. For example, U.S. Pat.Nos. 5,492,113; 5,551,418; 5,904,141; 5,823,187; and 5,901,704 all toEstes et al., the contents of which are incorporated herein byreference, describes a pressure ramp technique in which activating theramp a first time causes the pressure support system to deliver apressure ramp having a first duration, and a second activation of theramp causes the system to deliver a pressure ramp having a secondduration, which is typically shorter than the first duration. Thesefeatures can be incorporated in to the operation of ramp controller 118to determine the shape and duration of each pressure ramp.

The present invention also contemplates setting the slope, or allowingthe user to set the slope, of the pressure increase rather than itsduration. This is similar to having the system or allowing the user toselect the shape or profile of the pressure increase. Once the slope isknown, set, or determined, the system increases the IPAP and EPAP alongthat slope until EPAP reaches EPAP_(min).

As noted above, the present invention contemplates allowing the IPAP,EPAP, or both to change during the ramp cycle, and, in particular,during the pressure increasing portion of the ramp cycle. FIG. 32illustrates a situation in which the IPAP pressure is increased at point710, while the EPAP pressure remains unchanged. This can occur, forexample, if snoring is detected and IPAP is selected or used as thecontrol pressure, or if the patient is deemed to be experiencing onlyhypopneas during the pressure increase. Because the EPAP pressure hasnot been altered, the duration of the ramp (pressure increase) is notaltered by this change.

The present invention contemplates that the rate or slope of theincrease in pressure resulting from the ramp, returns to the value ithad prior to the increase. The slope of the IPAP 714 before point 710 isthe same as the slope of IPAP 716 after this pressure increase. Inaffect, pressure increase 718 is merely a brief change in the slope ofthe IPAP waveform during the pressure increase portion of the rampcycle. The dashed line in FIG. 32 represents the waveform the IPAP wouldhave had if pressure increase 718 has not occurred. The presentinvention contemplates that, the shape and duration of pressure increase718 is also selectable, controllable, or can be preset so that it canhave any desired configuration in addition to the relatively linearincrease illustrated in the figures.

In addition, the present invention contemplates maintaining the durationof the ramp to a fixed value, despite therapy pressure increases. Inthis case, if for example, the EPAP is increased, the slope of the rampis recalculate so that the ramp pressure increase terminates at the samepoint it would have but for the EPAP increase. The IPAP increase can belikewise recalculated.

FIG. 33 illustrates a situation in which both the IPAP and the EPAP areincreased at a point 712 during the pressure increase portion of theramp cycle. That is pressure increases 720 and 722 take place in theIPAP and EPAP waveforms, respectively, during the pressure ramp. Thiscan occur for example, if the IPAP pressure is increased and the EPAPlevel must also be increased to maintain the proper PS level, or if theEPAP pressure is increased and the IPAP level must also be increased tomaintain the proper PS level. Examples of situations that would resultin these types of pressure increases are discussed herein.

As in the previous embodiments, the rate or slope of the IPAP and EPAPwaveforms is the same after pressure increase 720, 722 as it was beforethe pressure increase. Because the EPAP level is increased by pressureincrease 722, the EPAP level will reach EPAP_(min) at point 724 which issooner than if the pressure increase had not taken place (point 726).Thus, pressure increase 722 effectively shortens the duration of theramp cycle.

While the ramp cycle is measured with respect to the EPAP pressure,i.e., the pressure increase begins when EPAP=EPAP_(Ramp) and ends whenEPAP=EPAP_(min), it is to be understood that the present inventioncontemplates that the IPAP pressure could also be used as the controlpressure, rather than EPAP. It is to be further understood that theIPAP, EPAP, or both can decrease during the ramp increase portion of theramp cycle. In the embodiment, of FIG. 33, if the EPAP level decreases,the result would be to effectively lengthen the duration of the rampcycle.

D. Flow Limit Control Layer

Flow limit control (FLC) layer, which is assigned a third (3rd)priority, includes an FLC detection module 120 that receives the flowsignals from flow sensor 46. FLC detection module 120 compares the totalflow Q_(total) (Q_(total)=Q_(patient)+Q_(leak)) to an empiricallydeveloped pressure versus flow curve 124 to determine if a patientdisconnect condition, such as a gross system leak or a mask offcondition, is occurring. FIG. 3 illustrates a pressure-flow diagram usedfor this comparison.

As shown in FIG. 3, the operating pressure (horizontal axis) for thepressure support system is either measured via pressure sensor 48 or isknown, because the pressure support system knows what pressure it isattempting to deliver to the patient. Pressure-flow curve 124 representsthe various flows for each operating pressure level that, if met orexceeded, represent a patient disconnect condition. In other words, FLCdetecting module 120 plots the total flow Q_(total), which is directlymeasured by flow sensor 46 as the flow in patient circuit 34, for theknown operating pressure on the chart shown in FIG. 3. If the total flowlies on or above curve 124, as indicated by points 126 and 128, the FLCdetector deems a patient disconnect condition to exist. Thus, it isassumed that patient interface device 38 has become disconnected fromthe patient or some other disconnect condition of the patient circuithas occurred. If, however, the total flow Q_(total) lies below curve124, as indicated by point 130, FLC detecting module 122 deems there tobe no patient disconnect condition.

In an exemplary embodiment of the present invention, the currentmeasured outlet pressure and the measured total flow is used to in theFLC layer, independent of whether the patient is in the inspiratory orthe expiratory phase of the breathing cycle. Other embodiments may usejust the IPAP or the EPAP or a combination of IPAP and EPAP, such as anaverage pressure across the breath, in the FLC control layer.

It can be appreciated that the location of pressure-flow curve 124 inthe pressure-flow chart is specific to the hardware used in the pressuresupport system. For example, a longer patient circuit introduces agreater pressure drop, and, hence, a different pressure flowrelationship that would indicate a patient disconnect condition, thanthat present in a pressure support system with a shorter patientcircuit. As noted above, the pressure flow relation 124 is preferablyempirically determined for the specific pressure support system. Ofcourse, a number of empirical relationships can be determined inadvance, with the specific relationship being selected when the systemcomponents are assembled.

Referring again to FIG. 2, if a patient disconnect condition is detectedby FLC detector 120, this indication is provided by FLC monitoringmodule 120, which monitors the duration that the patient flow is aboveFLC curve 124. If the total flow is at or above FLC curve 124, asindicated by the output of FLC detector 120, for a predetermined periodof time, such as 7 seconds, a request for control is sent to requestprocessor 106. The request from FLC monitoring module 122 is assignedthe third highest priority, and all other control operations, other thanthe on/off control 114 and ramp control 118, are overridden, so thatcontrol is given over to a FLC controller 132.

The purpose of the seven second time delay is to ensure that deepinhalations by a patient, which may cause the total flow to move outsidethe FLC curve temporarily, are not erroneously considered as a patientdisconnect condition. It can be appreciated that other duration timedelays can be used so long as temporary, patient induced flows are noterroneously deemed to be a disconnect condition. The present inventionfurther contemplates that if the FLC condition exists for a relativelylong period of time, such as 90 seconds, it is assumed that the patienthas removed the patient interface device. In which case, the system willautomatically turn itself off via well known auto on/off techniques.See, e.g., U.S. Pat. No. 5,551,418 to Estes, et al., which teachestechniques for automatically turning a pressure support system off or ondepending on whether the patient is using the system.

FLC controller 132, once activated, causes the IPAP and EPAP deliveredto the patient to be lowered to a low level that allows the user tocorrect the disconnect condition without having to fight thepressure/flow that would otherwise be delivered by the pressure supportsystem. In an exemplary embodiment, the IPAP and EPAP are lowered suchthat PS remains constant. This is accomplished, for example, bycontrolling the IPAP as the primary pressure and setting the EPAP levelbased on the PS level. These lower IPAP and EPAP pressure levelsdelivered by FLC controller 132 should be low enough to allow thepatient to reapply the mask without discomfort, yet high enough to allowthe pressure support system to detect when the patient has reapplied themask.

FLC controller 132 also causes the pressure generating system tocontinue to deliver the flow of breathing gas at these lower pressurelevels until the disconnect condition is corrected, i.e., until themeasured total flow Q_(total) falls below curve 124 so that control isno longer requested by FLC monitoring module 122, or until a time periodthat initiates the auto-off function elapses. In a preferred embodimentof the present invention, when the patient disconnect condition iscorrected, FLC controller 132 ramps the IPAP and EPAP levels deliveredto the patient back up to prior pressure levels to provide normal flow.

E. Snore Control Layer

Snore control layer, which is assigned a fourth (4th) priority, includesa snore detection module 140 that receives inputs from monitoring system44, such as pressure sensor 48 and/or flow sensor 46, and determinesfrom this information whether the patient is experiencing a snore. Thepresent invention contemplates that the decision as to whether thepatient is experiencing a snore can be made using any conventional snoredetection technique, such as those described in U.S. Pat. Nos.5,203,343; 5,458,137; and 6,085,747 all to Axe et al. However, in apreferred embodiment of the present invention, the determination ofwhether the patient is experiencing a snore is made according to theteachings of U.S. patent application Ser. No. 10/265,845 Truschel etal., the contents of which are incorporated herein by reference.

The present invention also contemplates further discriminating the snoreevent based on whether the snore event occurs in the inspiratory or theexpiratory phase of a respiratory cycle. During either phase of therespiratory cycle, the threshold above which a snore event wouldnominally be declared can be dependent upon some additionalparameter(s), such as that measured by pressure sensor 48 in FIG. 1. Forhigher pressures, the snore detection threshold could be raised, thusmaking it more difficult to detect a snore event. The value of the snoredetection threshold could be independently settable for each respiratoryphase. This implies that for a specific parameter, such as pressure, thethreshold at which a snore event would be declared for the inspiratoryphase could be either higher, lower, or the same as the thresholdsetting for the expiratory phase. It is to be understood that otherparameters, in addition to pressure, or a combination of parameters,could be used to set the threshold at which a snore event is declared.

Snore detection module 140 provides an output to snore monitoring module142 each time a snore event is declared. Snore monitoring module 142determines, based on the detected snore events, whether to initiate arequest for control of the pressure support system from requestprocessor 106. According to a presently preferred embodiment, snoremonitoring module 142 includes a counter that counts the number of snoreevents and a timer to measure the length of time between snore events.If a snore event does not occur within 30 seconds of the last snoreevent, then the counter is reset to zero. If the counter reaches three,a request for control is sent to request processor 106. Thus, if threesnore events occur, where each snore event is not longer than 30 secondsfrom the last snore event, a request for control is initiated. Thisrequest expires after 30 seconds and the snore counter in snoremonitoring module 142 is reset.

The request from snore monitoring module 142 is assigned the fourthhighest priority, and all other control operations, other than theon/off control 114, ramp control 118, and the FLC control 132, areoverridden, so that control is given over to a snore controller 144. Ifthe request process results in control being given to snore controller144, the snore controller causes pressure generating system 32 to raisethe EPAP level delivered to the patient by a predetermined amount, suchas 1.0 cmH₂O. In a preferred embodiment, this pressure increase is doneat a rate of 1 cmH₂O per 15 seconds. In this exemplary embodiment, theIPAP is not changed. As a result, that the PS level will decrease, solong as the PS level is greater than or equal to the PS_(min). If itshould happen that the PS level reaches PS_(min) as a result ofincreases EPAP toward IPAP, the PS level is held constant at PS_(min).In which case, further increases in the EPAP will also result in anincrease in IPAP in order to keep the PS level at or above PS_(min).

Snore controller 144 releases control, and as a background task, sets upa one minute lockout interval. The IPAP and EPAP levels at the end ofthe pressure increase are stored as snore treatment IPAP and EPAPlevels, respectively. It is believed that these snore treatment IPAP andEPAP levels represents pressure levels that provides a relatively goodtreatment to the patient to treat many of the breathing disorders he orshe may experience.

The lockout interval also prevents the pressure support system fromattempting to over-treat the patient with another EPAP increase if, forexample, additional snore events occur that would otherwise cause thesnore controller to increase pressure. If, however, additional snoreevents occur that meet the above-described criteria required by snoremonitoring module 142 and the lockout interval has elapsed, the snoremonitoring module will again request control and, if granted, snorecontroller 144 will again increase IPAP and EPAP (up to the maximumpressure set point). These new IPAP and EPAP levels are stored as thesnore treatment pressures.

In the embodiment described above, the EPAP level is the controlledpressure. However, the present invention also contemplates that the IPAPcan be controlled by the snore controller. That is, if control is givento snore controller 144, the snore controller causes pressure generatingsystem 32 to raise the IPAP level delivered to the patient by apredetermined amount, such as 1.0 cmH₂O. In a preferred embodiment, thispressure increase is done at a rate of 1 cmH₂O per 15 seconds. In thisexemplary embodiment, the EPAP is not changed. As a result, that the PSlevel will increase, so long as the PS level is does not exceedPS_(max). If it should happen that the PS level reaches PS_(max) as aresult of increases IPAP away from EPAP, the PS level is held constantat PS_(max). In which case, further increases in the IPAP will alsoresult in an increase in EPAP in order to keep the PS level at or abovePS_(max).

It is to be understood that the number of snore events used in snoremonitoring module 142 to determine when to request control of thepressure support system, the amount and rate of the IPAP and EPAPincrease provided by snore controller 144, and the duration of thelockout can be varied.

F. Big Leak Control Layer

The big leak control layer, which is assigned a fifth (5th) priority, issomewhat similar to the FLC control layer in that this control layeranalyzes the estimated patient circuit leak Q_(leak) and compares it toanother empirically developed pressure versus flow curve. However, thiscontrol layer is not attempting to determine whether the patient hasremoved the patient interface device or whether a patient circuitdisconnection or other gross leak event has occurred. Rather, the bigleak control layer attempts to determine when the estimated leak fromthe system exceeds a reliable range of operation.

Big leak control layer, includes a big leak detection module 150 thatreceives the flow signals from flow sensor 46. Big leak detection module150 determines the estimated leak Q_(leak) from this signal using anyconventional leak estimation technique and sends this information to bigleak monitoring module 152. In big leak monitoring module 152, theestimated leak is compared to an empirically developed curve todetermine if the leak from the system exceeds a worse case leak.

Referring again to FIG. 3, the operating IPAP (horizontal axis) for thepressure support system is known. Curve 154 represents the various flowsfor each operating IPAP level that, if exceeded, represent a leak thatis larger than the worst case system leak. In other words, big leakmonitoring module 152 plots the estimated leak Q_(leak) for the knownoperating IPAP on the chart shown in FIG. 3. If the estimated leak isabove curve 154, as indicated by points 126, 128, and 130, the estimatedleak exceeds the leakage flow that constitutes a reliable operatingrange for the pressure support system. This can occur, for example, ifthe patient interface device becomes partially dislodged from thepatient so that more gas is leaking from the patient circuit than wouldotherwise be expected for the type of patient circuit being used. If,however, the estimated leak Q_(leak) lies on or below curve 154, asindicated by points 156 and 158, big leak monitor 152 deems there to bean acceptable level of system leak.

A further embodiment of the present invention contemplates using anaverage operating pressure in determining whether the leakage rateconstitutes a big leak. The present invention also contemplates usingthe IPAP, EPAP, or some other combination of IPAP and EPAP as thereference pressure for determining whether there is a big leak.

It can be appreciated that the specific location of curve 154 in thepressure-flow chart is specific to the hardware used in the pressuresupport system. For example, different size exhaust devices that allowdifferent exhaust flows would require different pressure-flowrelationships. Pressure-flow relation 154 is preferably empiricallydetermined for the specific pressure support system. Of course, a numberof empirical relationships can be determined in advance, with thespecific relationship being selected when the system components areassembled.

Referring again to FIG. 2, if a big leak condition is detected by bigleak monitoring module 152, a request for control is sent to requestprocessor 106. As noted above, the request from big leak monitoringmodule 152 is assigned the fifth highest priority, and all other controloperations, other than on/off control 114, ramp control 118, FLC control132, and snore control 144, are overridden, so that control is givenover to big leak controller 162.

Once control is given to big leak controller 162, this controller causesthe IPAP delivered to the patient by pressure generating system 32 to belowered by a predetermined amount, at a predetermined rate, for apredetermined period of time. In this exemplary embodiment, the EPAP isnot changed. As a result, the PS level will decrease when IPAP islowered so long as the PS level is greater than or equal to thePS_(min). For example, a presently preferred embodiment of the presentinvention contemplates reducing the IPAP delivered to the patient by 1cmH₂O over a period of 10 seconds and holding at this new pressure for 2minutes. If it should happen that the PS level reaches PS_(min), the PSlevel is held constant at PS_(min). As a result, the EPAP will also bedecreased in conjunction with any further decreases in IPAP in order tokeep the PS level at or above PS_(min).

Big leak detection module 152 will continue to request that big leakcontroller assume control of the pressure support system so long as thecriteria necessary to satisfy the big leak monitoring module are met. Ifthe request is again granted, after the 2 minute hold, the big leakcontroller would repeat the IPAP reduction and hold process until thebig leak condition is resolved or a minimum IPAP or EPAP is reached. Thebig leak condition must also clear for a predetermined period of, suchas 90 seconds, before control is released by this control layer.

One potential result of the big leak control layer is that this IPAPdrop may arouse the patient at least slightly. It is believed that thebig leak condition will be resolved if this arousal causes the patienteither to roll over and inadvertently reposition the mask or wake up andadjust the mask. It is also believed that by lowering the IPAP, thepatient interface device may reseat itself, thereby eliminating the bigleak condition.

G. Apnea/Hypopnea Control Layer

Apnea/hypopnea (A/H) control layer, which is assigned a sixth (6th)priority, includes an A/H detection module 164 that receives inputs frommonitoring system 44, and, in particular flow sensor 48, and determines,from this information, whether the patient is experiencing an apnea or ahypopnea. This determination is provided to A/H monitoring module 166that decides whether to request that an A/H control module 168 takecontrol of the pressure generating system.

The present invention contemplates that A/H detection module 164monitors the variation of the inspiratory peak flow, referred to as theweighted peak flow (Q_(Wpeak)), and determines from the weighted peakflow, as discussed in detail below, whether the patient is experiencingan apnea or hypopnea. Thus, in order to understand the operation of theA/H control layer, it is necessary to first understand how the presentinvention determines the weighted peak flow (Q_(Wpeak)).

1. Weighted Peak Flow

FIG. 5 is a graph of an exemplary inspiratory waveform 170 of thepatient flow, and FIGS. 4A-4C are graphs illustrating the differencebetween an actual peak flow and a weighted peak flow Q_(Wpeak) used bythe present invention. As shown in FIGS. 4A-4C, which illustratedifferent exemplary inspiratory waveforms 172, 174 and 176,respectively, the actual peak Q_(peak) is the high point on theinspiratory waveform. It can be appreciated from FIGS. 4A-4C that thehighest peak flow may be of little clinical value. For example, in FIG.4C the peak flow is exaggerated due to the flow overshoot at the startof inspiration. For this reason, the present invention does not useQ_(peak). Instead, the present invention uses the weighted peak flowQ_(Wpeak), the approximate location of which is shown by the dashedlines in FIGS. 4A-4C.

Referring now to FIG. 5, to determine Q_(Wpeak) for an inspiratorywaveform, such as flow waveform 170, the present invention firstdetermines a start point 180 and a stop point 181 for the inspiratorywaveform. This is accomplished using any conventional technique. Thetotal volume of the inspiratory flow is then calculated. Again, this canbe accomplished using any conventional technique. Next, the systemdetermines the points on the inspiratory waveform that correspond to the5% volume (point 182), 20% volume (point 184), 80% volume (point 186),and 95% volume (point 188). The next steps require determining twobaseline levels, a Flatness Round Baseline (FRB) and a RoundnessBaseline (RB).

The Flatness Roundness Baseline (FRB) is determined by comparing all ofthe flow values of the points on the waveform between the 5% and the 95%volume points against the flow values at the 5% and 95% volume points.This is done to find the lowest point from among the range of pointsbetween 5% and 95%, which is used to set the FRB. A line drawn at thelowest point from among these points defines the FRB.

The Roundness Baseline (RB) is determined by comparing all of the flowvalues of the points on the waveform between the 20% and the 80% volumepoints against the flow values at the 20% and 80% volume points. This isdone to find the lowest point from among the range of points between 20%and 80%, which is used to set the RB. A line drawn at the lowest pointfrom among these points defines the RB.

The system also calculates two further baselines; a Flatness FlatBaseline (FFB) and a Flatness Baseline (FB), based on the FlatnessRoundness Baseline (FRB) and the Roundness Baseline (RB), respectively.More specifically, the FFB is determined as the average of all flowmeasurements above the FRB and between the 5% and 95% volume points. Inmost cases, this will correspond to the flow measurements between the 5%and the 95% volume points, as shown in FIG. 5. However, it is possiblefor the FRB to be below the 5% or 95% volume shown in FIG. 5. It can beappreciated that finding the average of the flow measurements from thestart to the end of the FRB line is equivalent to determining the volumeof areas A and B in FIG. 5 and dividing this volume by the period oftime (T_(5%-95%)) between the 5% volume and the 95% volume.

The Flatness Baseline (FB) is determined as the average of all flowmeasurements above the RB and between the 20% and 80% volume points. Inmost cases, this will correspond to the average of all flow measurementsbetween the 20% and the 80% volume points. However, it is possible forthe RB to be below the 20% or 80% volume shown in FIG. 5. It can beappreciated that finding the average of the flow measurements from thestart to the end of the RB line is equivalent to determining the volumeof area B in FIG. 5 and dividing this volume by the period of time(T_(20%-80%)) between the 20% volume and the 80% volume. The FlatnessBaseline level is the weighted peak flow Q_(Wpeak).

2. Apnea/Hypopnea Detection Criteria Modeling

Apnea/hypopnea detection module 164 gathers weighted peak flow Q_(Wpeak)information over a period of time to determine a model weighted peakflow Q_(WPM), which is used for comparison purposes in performing thehypopnea and apnea detection processes discussed below. In particular,A/H detector 164 monitors the weighted peak flows for the inspiratorybreaths occurring over a 4 minute moving window. The present inventionfurther contemplates that the duration of this window can be a durationother than four minutes, such as 1-6 minutes. These weighted peak flowsare statistically sorted as shown in FIG. 6, which is an exemplaryhistogram of the weighted peak flows for the breaths accumulated duringthe moving window.

In one embodiment of the present invention, the model peak weighted peakflow Q_(WPM) is determined as the weighted peak flows falling at the85th percentile of the accumulated weighted peak flows. However, in apreferred embodiment of the present invention, the weighted peak flowsfalling between the 80th and 90th percentiles are averaged, and thisaverage value is taken as the model peak weighted peak flow Q_(WPM).

3. Hypopnea Detection

FIGS. 7A-7E are flow charts illustrating the hypopnea detection processcarried out by A/H detector 164 according to the principles of thepresent invention. The hypopnea detection begins in step 190 where adetermination is made as to whether the model weighted peak flow Q_(WPM)exists. The model weighted peak flow Q_(WPM) can be reset, for example,if a high leak level or rapid changes in the leak level are detected. Inwhich case, there would not be enough information from which todetermine whether the patient is experiencing a hypopnea. Therefore, ifthere is not enough data to generate the model weighted peak flowQ_(WPM), the system continues to collect data to generate thisinformation. If the model weighted peak flow Q_(WPM) exists, the systemmoves to step 192.

In step 192, an arming threshold is determined. The purpose of thearming threshold is to ensure that the patient has at least onerelatively large breath going into the hypopnea. This relatively largebreath should have a weighted peak that is outside the hypopneadetection range, so that smaller breaths that are within this range canbe detected. Without first finding a breath that is outside the hypopneadetection range, it would be difficult, for example, to determinewhether the patient has started a new hypopnea or is merely continuingan existing hypopnea. In an exemplary embodiment of the presentinvention, the arming threshold is set to 72% of the current modelweighted peak flow Q_(WPM). Other arming thresholds are contemplated,such as ranging from 55%-75% of the current Q_(WPM).

In step 194, the current weighted peak flow Q_(Wpeak) is compared to thearming threshold to look for the relatively large entry breath. If nosuch breath is detected, i.e., if the current weighted peak flowQ_(Wpeak) is less than the arming threshold, the system returns to step190 and this process repeats. If, however, a breath having a weightedpeak flow Q_(Wpeak) that is outside the arming threshold is detected,the system moves to step 200.

In step 200, the hypopnea detection threshold is determined as 66% ofthe model weighted peak flow Q_(WPM). Other hypopnea detectionthresholds are contemplated, such as ranging from 45%-70% of the currentQ_(WPM). In step 202 a weighted peak flow Q_(Wpeak) for a currentinspiratory phase is compared to the detection threshold calculated instep 200. If the current weighted peak flow Q_(Wpeak) is greater than orequal to 66% of the model weighted peak flow Q_(WPM), the system returnsto step 200. If, however, the current weighted peak flow Q_(Wpeak) isless than 66% of the model weighted peak flow Q_(WPM), the system movesto step 204 and begins monitoring for the occurrence of a hypopneaevent.

In step 204, the model weighted peak flow Q_(WPM) at the start of thehypopnea detecting is clamped or latched for use in determining otherthresholds. This clamped value Q_(WPMclamped) for the model weightedpeak flow Q_(WPM) is used to determine a hysteresis level. Thehysteresis level is set to 72% of Q_(WPMclamped) and the system moves tostep 206. Other hysteresis levels are contemplated, such as ranging from60%-80% of Q_(WPMclamped). The clamped value Q_(WPMclamped) is also usedto set a first termination threshold, which is the weighted peak flowQ_(Wpeak) that must be met by a monitored inspiratory waveform in orderto terminate the hypopnea detection process. The first hypopneatermination threshold is set at 78% of Q_(WPMclamped). Other hypopneatermination thresholds are contemplated, such as ranging from 70%-85% ofQ_(WPMclamped). In step 204 a new arming threshold is calculated. Thisis done because the arming threshold calculated in step 192 may nolonger be valid, especially if a significant amount of time has passedsince the arming threshold was calculated in step 192. The armingthreshold is set to 72% of the current model weighted peak flow Q_(WPM).

In step 206 a decision is made whether to stop the hypopnea monitoringprocess. This may occur, for example, if a discard event occurs or ifthe weighted peak flow exceeds the hysteresis level. A discard eventoccurs, for example, when the data provided to the detection moduleincludes an aberration or is incomplete. If the hypopnea monitoringprocess stops in step 206, the system, in step 207, checks the currentweighted peak flow Q_(Wpeak) against the arming threshold, which is thearming threshold calculated in step 204. If the current weighted peakflow Q_(Wpeak) is greater than the arming threshold, the system returnsto step 200. If the current weighted peak flow Q_(Wpeak) is less than orequal to the arming threshold, the system returns to step 190.

The reason for returning to step 200, rather than step 190 if thecurrent weighted peak flow Q_(Wpeak) is greater than the armingthreshold, is because the patient is already having breaths that arelarge enough to determine that a hypopnea is occurring. Thus, there isno need to recalculate the arming threshold, so instead, the systemreturns to step 200 to begin looking for a hypopnea.

If the hypopnea monitoring process continues from step 206, the systemdetermines in step 208 whether a sufficient amount of time has elapsedwith the weighted peak flow Q_(Wpeak) being below the hypopnea detectionthreshold and whether a sufficient number of breathing cycles haveoccurred. In the presently preferred exemplary embodiment, the weightedpeak flow must be below the hysteresis threshold for at least 10 secondsand there must be at least one detectable breathing cycle in order to beconfident that the patient is experiencing a hypopnea. Thus, in step208, a determination is made whether 10 seconds have elapsed and whetherone breathing cycle having non-zero peak flow levels has occurred. Ifnot, the system returns to step 206. If so, the system begins monitoringfor a first termination breath in step 210. The first termination breathis a breath that ends the hypopnea event.

During the hypopnea event, i.e., once the hypopnea monitoring began instep 204, the minimum weighted peak flows were being monitored. In step210, the lowest minimum weighted peak flow that has been detected so faris identified. This value is then doubled and used as a second hypopneatermination criteria in monitoring for the first termination breath. Thepurpose of this second hypopnea termination criteria is to allow largedeviations from the relatively low peak levels that occur during ahypopnea to terminate the hypopnea monitoring process. Please alsorecall that the first hypopnea termination criteria was determined instep 204 as 78% of Q_(WPMclamped).

In step 212, a decision is made whether to stop the hypopnea detectionprocess. This will occur if, for example, a discard event occurs or ifthe hypopnea has lasted beyond a duration normally associated with atrue hypopnea event. In the presently preferred embodiment, thisduration is 60 seconds. Thus, in step 212, the system determines whetherthe hypopnea conditions have been met for more than 60 seconds. If so,the hypopnea detection process is stopped, all logic flags are reset,and the process returns to step 190. If the hypopnea detection processcontinues, a determination is made in step 214 whether the weighted peakflow for the current breath meets the first or second terminationthreshold.

If the weighted peak flow for the current breath is greater than 78% ofQ_(WPMclamped) (first hypopnea termination criteria) or if the weightedpeak flow for the current breath is greater than two times the lowestnon-zero weighted peak flow (second hypopnea termination criteria), avalid first termination breath is declared, and the system processes tostep 216. If a valid first termination breath is not detected in step214, the system returns to step 210 and continues to monitor for a firsttermination breath.

Once a first termination breath is detected in step 214, the nextproceeding breath must meet a third hypopnea termination threshold,which is determined in step 216. The third hypopnea terminationthreshold is set, in step 214, at 80% of the minimum of the first andsecond termination criteria thresholds.

In step 218, it is determined whether the weighted peak flow of nextbreath immediately after the first termination breath is 80% of theminimum of the first and second termination criteria thresholds. If so,the hypopnea monitoring process is terminated and a hypopnea is declareddetected in step 220. If not, the hypopnea detection process is stopped,all logic flags are reset, and the process returns to step 200. Thethreshold against which the weighted peak flow of next breathimmediately after the first termination breath can be a value other than80%. For example, thresholds ranging from 70% to 100% are contemplatedby the present invention

In summary, in order to detect a hypopnea, the following criteria mustbe met:

a) Valid model weighed peak flow data Q_(WPM) must exist (step 190);

b) There must be an entry breath that is outside the hypopnea detectionrange (steps 192 and 194);

c) The weighted peak flow of a breath must fall below the hypopneadetection threshold (step 202);

d) The weighted peak flow of subsequent breaths must remain below thehysteresis threshold for at least 10 seconds and at least one breathmust be detected (steps 206 and 208);

e) The weighted peak flow of a breath must rise above the lesser of thefirst termination threshold or the second termination threshold (step214) and the next breath must be above a third termination thresholdwhich set based on the first and second termination thresholds;

f) The duration of the hypopnea event must not exceed 60 seconds (step212); and

g) A discard event must not occur (steps 206 and 212).

4. Apnea Detection

As with hypopnea detection, A/H detection module 164 determines whetherthe patient is experiencing an apnea by comparing the weighted peak flowQ_(Wpeak) for each breathing cycle to the model weighted peak flowQ_(WPM). More specifically, an apnea detection process starts if thecurrent weighted peak flow Q_(Wpeak) falls below 20% of the modelweighted peak flow Q_(WPM). When this occurs, the model weighted peakflow Q_(WPM) at the start of the apnea monitoring process is clamped orlatched. This clamped value Q_(WPMclamped) is also used to set an apneatermination threshold, which represents the weighted peak flow that mustbe met by a monitored inspiratory waveform in order to terminate theapnea detection process. The apnea termination threshold is set as 30%of Q_(WPMclamped). In this case, the apnea takes precedence andoverrides, resets, or temporarily disables the hypopnea detection.

Once an apnea monitoring process begins, if the weighted peak flowremains below the termination threshold for a predetermined period oftime, which in a preferred embodiment is 10 seconds, a start of apneaevent is declared. It should be noted that both the hypopnea and apneadetection take place concurrently.

The present inventors appreciated that during an apnea event, thepatient may sometimes make a momentary respiratory effort. FIG. 8illustrates an exemplary patient flow waveform 222 in which an apnea 224begins generally at 226 and terminates generally at 228. During apnea224, the patient made a respiratory effort having very short duration,yet relatively high peak flow, identified as respiratory bursts 230.During periods 232, before and after these bursts, the patient flow wasat a relatively low level typical of an apnea. The present inventioncontemplates effectively ignoring transient bursts 230 in monitoring forthe occurrence of an apnea. If these bursts are not ignored, there is achance that an apnea detector could erroneously consider the burst, and,thus, disregard this sequence as an apnea.

5. Apnea/Hypopnea Monitoring

The occurrence of a hypopnea event and the start of an apnea event arereported by A/H detector 164 to A/H monitor 166, which then mustdetermine whether to request that A/H controller 168 take control of thepressure generating system. In a presently preferred embodiment of theinvention, A/H monitor 166 will issue a request for control to requestprocessor 106 if two apnea events or if two hypopnea events, asdetermined in the manner discussed above by A/H detector 164, occurwithin a predetermined period of time. In a presently preferredembodiment, this period of time is a three minute moving window.However, those skilled in the art can appreciate that the period forthis window can be varied.

The present invention also contemplates causing A/H monitor 166 to issuea request for control to request processor 106 if a mixture of apneaevents and hypopnea events occur. For example, if two apnea or hypopneaevents occur within a predetermined period of time, A/H monitor wouldissue the control request.

6. Apnea/Hypopnea Pressure Control

Once A/H controller 168 is granted control, the determination of how tocontrol the IPAP and EPAP delivered to the patient is made based onwhether the patient experienced apneas, hypopneas, or a combination ofapneas and hypopneas.

If the patient is deemed to have been experiencing hypopneas only, A/Hcontroller 168 initially gradually raises the IPAP 1 cmH₂O and holds thepressure at this level for 30 seconds. The EPAP level, however, is notchanged. As a result, the PS level will increase, so long as the PSlevel is not already at PS_(max). After the 30 second hold period, thecontroller then releases control (usually to an auto-titrationcontroller hold state discussed below). If the criteria for grantingcontrol to A/H controller 168 are met again, and if the patient is againdeemed to have been experiencing only hypopneas, the controller repeatsthis process and raises the patient IPAP 1 cmH₂O and executes the 30second hold, again without changing the EPAP level so long as the PSlevel is not already at PS_(max). A/H controller 168 can increase theIPAP to a predetermined amount, such as 8-12 cmH₂O, without restriction.If apneas or hypopneas are detected at pressures greater than thispredetermined amount, e.g., 11 cmH₂O, an additional pressure controlrestriction is invoked as described below.

If the patient is deemed to have been only apneas or a combination ofapneas and hypopneas, A/H controller 168 increases the EPAP 1 cmH₂O andholds the pressure at this level for 30 seconds. However, the IPAP levelis not changed. As a result, the PS level will decrease, so long as thePS level is not already at PS_(min). After the second hold period, thecontroller then releases control (usually to an auto-titrationcontroller hold state discussed below). If the criteria for grantingcontrol to A/H controller 168 are met again, and if the patient is againdeemed to have been experiencing only apneas or a combination of apneasand hypopneas, the controller repeats this process and raises thepatient EPAP 1 cmH₂O and executes the 30 second hold, again withoutchanging the IPAP level so long as the PS level is not already atPS_(min). A/H controller 168 can increase EPAP without restriction up tothe level that would cause IPAP to exceed the predetermined amount asdescribed above. If apneas or combination of apneas and hypopneas aredetected at IPAP greater than an predetermined amount, an additionalpressure control restriction is invoked as described below.

In the embodiment of the present invention described thus far, A/Hdetection module 164 cannot detect the difference between obstructiveapnea/hypopnea events and central apnea/hypopnea events, but compensatesfor this using A/H controller 168. More specifically, the A/H controlleris limited or in some cases restricted from increasing the pressure ifthe pressure is already above a threshold. Obstructive events can beresolved by increasing pressure. However, it is generally believed thatcentral apneas are not responsive to pressure increases. Therefore, ifthe IPAP or EPAP was increased as a result of the occurrence of anapnea, and further apneas occur, it is assumed that the apneas that areoccurring at the relatively high pressure, e.g., 11 cmH₂O, are central,and not obstructive, apneas. In which case, additional IPAP or EPAPincreases are not desired.

To achieve this goal, A/H controller 168 sets a target apnea/hypopneatreatment limit when an apnea or hypopnea control request is made by A/Hmonitoring module 164. In a presently preferred embodiment, the targetapnea/hypopnea treatment limit is set at 3 cmH₂O above the IPAP beingdelivered to the patient when A/H monitor 164 initiated a controlrequest. If, however, the patient IPAP is 10 cmH₂O or less, the targetapnea/hypopnea treatment limit is set at 13 cmH₂O. Once set, the targetapnea/hypopnea treatment limit remains in place until a period of timeelapses where there are no new apnea/hypopnea events. The presentinvention currently contemplates setting this interval to 8 minutes, sothat if 8 minutes go by after the target apnea/hypopnea treatment limitwas set without any new apnea/hypopnea control requests, the targetapnea/hypopnea treatment limit is cleared.

If the patient is experiencing hypopneas only, the present inventionsets the target apnea/hypopnea treatment limit to a higher value, suchas 14 cm H₂O instead of a 10 cm H₂O. This is done because when the A/Hdetector 164 is identifying hypopneas only, it is more likely that thepatient is suffering from obstructive events, and not central events.Therefore, more aggressive treatment of the obstructive event is madepossible by the higher threshold.

Referring now to FIG. 9, which illustrates an example IPAP curve 236 forthe patient pressure generated by the pressure support system, if anapnea/hypopnea control request is granted at point 238, where thepatient is currently at 10 cmH₂O, the target apnea/hypopnea treatmentlimit 240 is set at 13 cmH₂O. A 1 cmH₂O IPAP or EPAP increase and 30second hold are then performed during an A/H treatment interval 242, andcontrol is released by A/H controller 168 at point 244. During interval246 control of the pressure support system is being handled by someother control module, such as the auto-titration module discussed below.For purposes of this example, the patient IPAP or EPAP was increasedduring this interval by 1 cmH₂O. At point 248, which is at an IPAP of 10cmH₂O, another apnea/hypopnea control request is granted, and anotherA/H treatment interval 242 occurs. At the end of this interval (point250) the patient IPAP is at 13 cmH₂O, which is the target apnea/hypopneatreatment limit 240.

If another apnea/hypopnea control request is made by A/H monitor 166 atpoint 250 or at any IPAP above the target apnea/hypopnea treatmentlimit, request processor 106 will still hand over control to A/Hcontroller 168, but A/H controller 168 is prevented from making furtherincreases in IPAP due to the current patient IPAP being at or above thetarget apnea/hypopnea treatment limit. Instead, A/H controller 168 willincrease the EPAP, until the PS_(min) is reached. If PS_(min) isreached, IPAP is at the target apnea/hypopnea treatment limit, andevents continue, the A/H controller will decrease IPAP by apredetermined amount, such as 2 cmH2O, to point 252 during a pressuredecrease interval 254. This decrease in IPAP will also cause a decreasein EPAP in order to maintain the PS at PS_(min).

At the end of interval 254, the IPAP is either held constant, asindicated by line 256, or is dropped again and held constant, asindicated by line 258. The decision of whether to hold or decrease theIPAP from point 252 is made by comparing the current IPAP, i.e., thepatient IPAP at point 252, with the snore treatment IPAP. If there is nosnore treatment IPAP stored in the system, which will be the case if thesnore controller has not been activated, the IPAP is held at line 256.If there is a snore treatment IPAP, and if the current IPAP is more thana predetermined amount above this snore treatment IPAP, such as morethan 2 cmH₂O above the snore treatment IPAP, A/H controller 168 willdecrease the IPAP to a level that is a predetermined amount higher thanthe snore treatment IPAP, and hold it at the lower level, as indicatedby line 258, during interval 260. The present invention decreases thepressure to 1 cmH₂O above the snore treatment IPAP.

A/H controller 168 maintains the patient IPAP and EPAP constant duringinterval 260 until a predetermined period of time has elapsed since thestart of the pressure decrease, i.e., since point 250. This hold-offperiod exists in order to allow the patient to stabilize. In a presentlypreferred embodiment, the IPAP and EPAP is held constant until 15minutes has expired since the start of the 2 cmH₂O decrease. The presentinvention contemplates setting the duration of the hold period to otherlengths of time, so long as the period of time is sufficient to allowthe patient to stabilize. This hold process may be interrupted and resetat any time by a higher level controller. At the end of the 15 minutehold, the target apnea/hypopnea treatment limit is cleared and controlis relinquished by A/H controller 160.

H. Variable Breathing Control Layer

The Auto-titration controller, which is described in the next section,relies on the ability to trend the steady rhythmic breath patternsassociated with certain stages of sleep. When a patient is awake, in REMsleep, or in distress, breathing tends to be more erratic and theAuto-titration trending becomes unstable. It is, therefore, important tointerrupt the Auto-titration controller if the patient's breathingpattern becomes too variable. In essence, the variable breathing controllayer keeps the Auto-titration control layer from being too erratic.

Referring back to FIG. 2, the variable breathing control layer, which isassigned a seventh (7th) priority, includes a variable breathingdetector 270, a variable breathing monitor 272, and a variable breathingcontroller 274. As described in greater detail below, the variablebreathing control layer performs statistical analysis on the scatter ofthe trended weighted peak flow data to detect unstable breathingpatterns or abrupt changes in patient response. When activated, variablebreathing control module 274 takes priority over the auto-titrationcontroller, so that when a valid variable breathing indication isprovided by variable breathing monitor 272, control of the pressuresupport system is turned over to the variable breathing controller. Inshort, activation of variable breathing control module 274 interruptsthe operation of the auto-titration controller when breathing becomesunstable and appropriately manages any necessary pressure changes.

1. Variable Breathing Detection and Monitoring

Variable breathing detection module 270 monitors the weighted peak flowsQ_(Wpeak) over a moving window, which in a presently preferredembodiment, is a four (4) minute window. The detection module in essencetrends four minutes worth of weighted peak flow information to determinewhether this information is becoming too erratic. FIGS. 10A and 10B aregraphs illustrating examples of the scatter of weighted peak flows. InFIGS. 10A and 10B, the weighted peak flows are relatively closelybunched around a trend line 276 in area 278 and is relatively scatteredfrom the trend line in area 280. Trend line 276 is a best-fit linedetermined using any conventional statistical analysis technique basedon the weighted peak flows data collected during the current 4 minutewindow. The primary difference between FIGS. 10A and 10B is that thetrend line in FIG. 10B is shown with a non-zero slope. This is done tohighlight the fact that the trend line is a best-fit line based on thecollected data points.

Variable breathing detection module 270 determines the standarddeviation of the weighted peak flow data collected during the monitoringwindow as indicated by dashed lines 282. It should be noted that thestandard deviation is calculated based on the best-fit trend line 276.It can be further appreciated that a standard deviation 284 is less inregion 278 than a standard deviation 286 in region 280, indicating thatthe weighted peak flow data is more variable in region 280.

The present inventors appreciated that using the standard deviationalone as a measure of the degree of variation in the weighted peak flowdata may not produce consistently correct results. This is so, becausethe standard deviation of the weighted peak flow data when the meanpatient flow is relatively low is not exactly comparable to the samestandard deviation for a higher mean patient flow. The presentinvention, therefore, seeks to normalize the standard deviation to themean patient flow, and then takes the mean flow into consideration whenanalyzing the variation in the data.

FIG. 11 is a chart illustrating a normalization curve 290 that describesthe relationship between the mean patient flow and an adjusted meanpatient flow. It can be appreciated from reviewing this figure thatthere is a linear region 292 in which the adjusted mean flow (verticalaxis) has a one-to-one match with the actual mean flow (horizontalaxis). If the patient's mean flow for the 4 minute window is withinregion 292, no adjustment to this mean flow is made. There is also afirst region 294 having a ½ to one relationship between the adjustedmean flow and the actual mean flow. Thus, if the actual mean flow fallswithin region 294, which is between 15 and 25 liters per minute (lpm),then an adjusted mean flow is calculated based on curve 290. There isalso a flat region 296 where the adjusted mean flow is clamped to abaseline value even if the actual mean flow is decreased. Thus, if theactual mean flow is less than 15 lpm, the adjusted mean flow is clampedat 20 lpm.

It is to be expressly understood that the specific shape of curve 290and the delineations between the various regions is subject tovariation. For example, although not illustrated, the present inventionfurther contemplates providing this clamping feature if the mean flowexceeds a predetermined value, such as in region 298.

A variable breathing number (VB#) is calculated as follows:

$\begin{matrix}{{{VB}\#} = {\frac{{standard}\mspace{14mu}{deviation}}{{adjusted}\mspace{14mu}{mean}\mspace{14mu}{flow}}.}} & (1)\end{matrix}$The end result of the variable breathing detection process carried outby variable breathing detection module 270 is this variable breathingnumber. The higher the VB#, the more variable the weighted peak flowdata.

The variable breathing number is provided by variable breathingdetection module 270 to variable breathing monitoring module 272, whichcompares this number to threshold values to determine when to requestthat variable breathing controller 274 take control from theauto-titration controller. FIG. 12 is a chart illustrating thehysteresis threshold criteria for declaring that the patient isexperiencing variable breathing and, hence for requesting control of thepressure support system.

As shown in FIG. 12, an upper threshold 300 and a lower threshold 302are set in advance. Preferably, the values of these thresholds aredetermined from empirical data. Variable breathing monitor 274 declaresthere to be variable breathing and issues a request for control torequest processor 106, when the variable breathing number (VB#),represented by line 304, exceeds upper threshold 300. This occurs atpoint 306 in FIG. 12. Variable breathing monitor 274 will continue todeem there to be variable breathing, and, hence, continue to requestcontrol, even if the VB# falls below upper threshold 300. In short, avariable breathing active indication is turned on at point 306 andremains on over region 308, until the VB# falls below lower threshold302 at point 310. While the variable breathing active indication is on,variable breathing monitor 274 issues a request for control of thepressure support from request processor 106.

Similarly, variable breathing monitor 274 will continue to deem there tobe no variable breathing, and, hence, will not request control, even ifthe VB# rises above lower threshold 302. That is, the variable breathingactive indication is turned off at point 310 and remains off over region312, until the VB# exceeds upper threshold 300, which occurs at point314.

2. Variable Breathing Pressure Control

Once variable breathing controller 274 has been granted control of thepressure support system, it takes some initial action based on theaction that the auto-titration controller discussed below is taking.After this initial action, it performs an independent pressure controloperation. FIG. 13 is a chart illustrating the pressure controloperation of the variable breathing control module of the presentinvention. In the exemplary embodiment, the IPAP level is used as theprimary pressure being controlled and the EPAP level is set based on thePS level, which is maintained between PS_(min) and PS_(max) duringvariable breathing pressure control. The EPAP level will only change ifnecessary to maintain a PS level that is between PS_(min) and PS_(max).Thus, for variable breathing pressure control, all references topressure control apply to the IPAP level.

As shown in FIG. 13, the IPAP control operation performed by variablebreathing controller 274 is subdivided into the following three regions:a) an active response region 320, b) a pressure hold region 322, and c)a slow ramp region 324. The IPAP control performed by variable breathingcontroller 274 in each of these regions is discussed in turn below. Itis to be understood that even though there appears to be discontinuitiesin the delivered pressure in FIG. 13, this is only due to the manner inwhich each region is illustrated. In practice, the IPAP at the end ofregion 320 is the start pressure for the pressure control that takesplace in region 322. Similarly, the IPAP at the end of region 322 is thestart pressure for the pressure control that takes place in region 324.

In region 320, column A illustrates the possible prior IPAP curves,i.e., the possible IPAP control actions being taken by the pressuresupport system before operation of the system was handed over tovariable breathing controller 274. Column B illustrates thecorresponding IPAP control curves that are produced by variablebreathing controller 274 based on the prior curves. In case #1, a priorIPAP level 326 is flat (not increasing, not decreasing). In which case,variable breathing controller 274 will cause the IPAP delivered to thepatient to remain at this level, as indicated by pressure curve 328.

In case #2, a prior IPAP level 330 is increasing. In which case,variable breathing controller 274 initially decreases the IPAP deliveredto the patient at a rate of 0.5 cmH₂O per minute, as indicated bypressure curve 332. The magnitude of the decrease is dependent on themagnitude of the increase in prior IPAP level 330. IPAP decrease 332 isintended to erase the prior IPAP increase 330 that possibly caused thevariable breathing. However, in an exemplary embodiment, the totaldecrease in pressure drop 332 is limited to 2 cmH₂O. After pressuredecrease 332, variable breathing controller 274 holds the pressuresteady, as indicated by pressure curve 334.

In case #3, a prior IPAP level 336 is decreasing. In which case, thevariable breathing controller initially increases the IPAP delivered tothe patient at a rate of 0.5 cmH₂O per minute, as indicated by pressurecurve 338. The magnitude of increase 338 is dependent on the magnitudeof the decrease in prior pressure 336. IPAP level increase 338 isintended to erase the prior IPAP decrease 336 that may have caused thevariable breathing. However, in an exemplary embodiment, the totalincrease in IPAP level 338 is limited to 2 cmH₂O. After IPAP increase338, variable breathing controller 274 holds the pressure steady, asindicated by pressure curve 340.

In a presently preferred embodiment, the duration during which pressureis provided according to the paradigms discussed above for region 320,column B, is set to 5 minutes. Thus, IPAP curve 328 (case #1), curve332-334 (case #2), or curve 338-340 (case #3) is provided for 5 minutesor until the variable breathing condition clears. Thereafter, the IPAPlevel is controlled according the pressure operations of region 322. Itis to be understood, however, that this duration can be varied over arange of durations.

In region 322, the IPAP level is either maintained at a constant value,as indicated by pressure curve 342 (case #4), or it follows a decreaseand hold pattern, as indicated by pressure curve 344 (case #5). Thedecision to hold the IPAP (case #4) or to decrease the IPAP (case #5) ismade by comparing the current IPAP, i.e., the patient IPAP at the end ofregion 320, with the snore treatment IPAP. This is similar to thepressure control operation of A/H controller 168 discussed above withrespect to FIG. 9.

If there is no snore treatment IPAP stored in the system, which will bethe case if the snore controller has not been activated, the IPAP isheld constant as pressure curve 342. If there is a snore treatment IPAP,and if the current IPAP is more than a predetermined amount above thissnore treatment pressure, such as more than 2 cmH₂O above the snoretreatment IPAP, variable breathing controller 274 decreases the IPAP toa level that is a predetermined amount higher than the snore treatmentIPAP, as indicated by IPAP curve 344, and holds the IPAP at the lowerlevel, as indicated by line 346, over the duration of region 322. Thepresent invention decreases the IPAP during IPAP decrease 344 to thesnore treatment pressure+1 cmH₂O.

In a presently preferred embodiment, the duration during which IPAP isprovided according to the paradigms discussed above for region 322 isset to 15 minutes. Thus, IPAP curve 342 (case #4) or curve 344-346 (case#5) is provided for 15 minutes or until the variable breathing conditionclears. Thereafter, the IPAP is controlled according to the pressureoperation of region 324. It is to be understood, however, that this 15minute duration can be varied over a range of durations.

In region 324, there is only one IPAP control operation. Namely, theIPAP level delivered to the patient is slowly ramped down, as indicatedby pressure curve 348. This downward pressure ramp continues until aminimum system pressure (IPAP_(min) or EPAP_(min)) is reached or untilthe variable breathing condition clears.

I. Auto-Titration Control Layer

The auto-titration control layer is assigned an eighth (8th) and lowestpriority from among all of the control layers. As a result, the pressurecontrol operations carried out by this layer are interrupted if anyother controller is activated. As shown in FIG. 2, the auto-titrationcontrol layer includes an auto-titration detection module 350, anauto-titration monitoring module 352, and an auto-titration controlmodule 354.

As will perhaps be better appreciated after reviewing the followingdiscussion of the auto-titration control layer, the various componentsof this layer interact very closely with one another. That is, while thepressure support system is operating in this control layer, theauto-titration detector and monitor are continuously analyzing theoutputs from monitoring system 44 because the output of theauto-titration monitor dictates how the auto-titration controlleradjusts the pressure at the patient. Unlike the other control layers,there is no need for the auto-titration monitoring module to requestcontrol from request processor 106, because the auto-titration controllayer is the default control layer, and will automatically be operatingif no other control layer has taken control.

The general goal of the auto-titration control layer is to induce slowpressure ramps, e.g., ±0.5 cmH₂O/min or to provide a pressure holdperiod, referred to as a therapy pressure. The patient's response tothese pressure changes and to the therapy pressure is evaluated bymonitoring certain parameters associated with the flow waveform todetermine whether the patient flow waveform is improving, degrading, orshowing no change. For each breath, values are calculated representingthe weighted peak flow Q_(Wpeak), roundness, flatness, and skewness ofthat breath. This data is stored and trended over time in a continuouseffort to optimize the pressure delivered to the patient by the pressuresupport system.

1. Peak, Roundness, Flatness and Skewness

As noted above, during the auto-titration control process carried out byauto-titration controller, the weighted peak flow Q_(Wpeak), roundness,flatness, and skewness of the inspiratory waveform for a breath aredetermined. Each of these characteristics of the inspiratory waveformare trended over time by auto-titration detector 350 to produced atrended value. This trended value is provided to auto-titrationmonitoring module 352, where it is used in a voting scheme discussed ingreater detail below to determine what action the auto-titrationcontroller takes. Therefore, it is important to understand first how thepresent invention calculates these inspiratory waveform characteristics.

The calculation of the weighted peak flow Q_(Wpeak) was discussed abovewith reference to A/H detection module 164. Therefore, no furtherexplanation of this inspiratory waveform characteristic is required.

In order to calculate the roundness characteristics of the inspiratorywaveform, the present invention compares a patient's inspiratory wave toa sine wave. FIGS. 14A-14C illustrates an exemplary patient inspiratorywaveform 360 including points 362 and 364 on this waveform thatcorresponds to the 5% and 95% volumes, respectively. Comparing waveform360 to a sine wave requires matching the patient's inspiratory wave tothe sine wave, or vice versa, in order to make the best possiblecomparison. For this reason, several steps must be taken in order to fitthe sine wave onto the patient's inspiratory waveform.

First, a sine base value, which is used to place the start and endpoints of a sine wave on patient's inspiratory waveform 360 iscalculated. The sine base value is defined as ½ of the flatness flatbaseline (FFB) value. Points 366 and 368 where line 370, which is a linecorresponding to the sine base (½FFB), intersects inspiratory waveform360 are selected as a start point and an end point of the sine wave tobe overlaid on the inspiratory waveform. The task then becomes locatingpoints 366 and 368 on waveform 360.

The present invention locates these points by searching for the pointson the inspiratory waveform beginning from a known landmark value, suchas the 5% volume point 362 and the 95% volume point 368. As shown inFIG. 14B, when searching at the start or proximal end of the inspiratorywaveform, if the flow value for the 5% volume (point 362) is less thanthe sine base value, search up, i.e., toward a distal end of waveform360, i.e., where the 95% volume point is located. On the other hand, ifthe flow value for the 5% volume (point 362) is greater than the sinebase value, search down, i.e., toward the proximal end or beginning ofwaveform 360. Arrow 370 in FIG. 14B indicates a downward search from the5% volume point toward the proximal end of the waveform, because, inthis exemplary embodiment, the flow at point 362 is greater than thesine base value.

When searching at the distal end of the inspiratory waveform, if theflow value for the 95% volume (point 364) is greater than the sine basevalue, search up, i.e., toward the distal end of waveform 360. On theother hand, if the flow value for the 95% volume (point 364) is lessthan the sine base value, search down, i.e., toward the proximalbeginning of waveform 360 where the 5% volume point is located. Arrow372 in FIG. 14B indicates a downward search from the 95% volume point,because, in this exemplary embodiment, the flow at point 364 is lessthan the sine base value.

In searching for the location of the points on waveform 360 thatcorrespond to the sine base value, it can happen that a search beginningat a landmark, such as the 5% volume point, fails to find the correctpoint on waveform 360 that should correspond to the start of the sinewave. For example, if point 362 is above the sine base value point andthe searching is done upward, as indicated by arrow 374 in FIG. 14C, thesearch for the start point may erroneously locate point 368, which isnear the end of the inspiratory waveform, as the start point. A similarerror would occur if the 95% point is greater than the pointcorresponding to the sine base value, shown as exemplary point 376, anda downward search was done from point 376, as indicated by arrow 378.

To avoid these errors, the present invention includes validity checks tosee if the search (arrows 374 and 376) crossed one another. If so, thepoints found by each search are discarded and no calculation ofroundness and flatness are made for that waveform. A similar error andresult occurs if no point is found that corresponds to the sine basevalue. This can occur, for example, if upward searching begins at point364, as indicated by arrow 380.

Once start point 366 and end point 368 for a sine wave template 382 inFIG. 15 are known, the amplitude (Sine Amp) of sine wave template 382having these start and end points is calculated using the knownrelationship between the width or period of a sine wave and itsamplitude. See FIG. 15. For example, the Sine Amp is calculated as:

$\begin{matrix}{{{Sine}\mspace{20mu}{Amp}} = {\frac{\int_{{Start}\mspace{14mu}{point}}^{{End}\mspace{14mu}{point}}{Q_{patient}(t)}}{2\;\pi}.}} & (2)\end{matrix}$

From the known period of the sine wave, i.e., the time between the startand end points, and the calculated amplitude, the present invention thendetermines a ratio of amplitude over period. In other words, a ratio iscalculated as:

$\begin{matrix}{{Ratio} = {\frac{{Sine}\mspace{14mu}{Amp}}{Period}.}} & (3)\end{matrix}$The purpose of determining this ratio is to attempt to normalize thesine wave templates to one another by adjusting the amplitude of thesine wave templates. For example, if the ratio is very high, itindicates that the sine wave template 384 is very tall and thin, asshown, for example, in FIG. 16A. If the ratio is very low, the sine wavetemplate 386 is very short and wide, as shown, for example, in FIG. 16B.It is preferable not to compare these tall, thin templates 384 or short,wide templates 386 to the actual patient inspiratory waveform becausethe fit between these two wave patterns is typically not very good anddoes not produce meaningful results.

To account for these conditions, the present invention adjusts the ratioof the sine wave template. FIG. 17 illustrates a normalization curve 390that is used to adjust the ratio of the sine wave templates.Normalization curve 390 includes a linear region 392 where no ratioadjustment is made. Above linear region 392, i.e., where the sine wavetemplate has a ratio that is too high, normalization curve 390 includesa first region 394 that downwardly adjusts the ratio and a clampingregion 396. In the illustrated exemplary embodiment, the adjusted ratiois clamped at 36, no matter how high the actual ratio is. Below linearregion 392, i.e., where the sine wave template has a ratio that is toolow, normalization curve 390 includes a second region 398 that upwardlyadjusts the ratio and a clamping region 400. In the illustratedexemplary embodiment, the adjusted ratio is clamped at 8 no matter howlow the actual ratio is.

The adjusted ratio determined from the relationship shown, for example,in FIG. 17, is used to set the amplitude of the sine wave template, withthe period being held constant. For example, FIG. 18A illustrates a sinewave template 402 where the ratio is too low, meaning that the sine wavetemplate is too flat. A corrected sine wave template 404 is also shownindicating how adjusting the ratio effectively increases the amplitudeof the sine wave template. FIG. 18B illustrates a sine wave template 406where the ratio is too high, meaning that the sine wave template is tootall. A corrected sine wave template 408 is also shown indicating howadjusting the ratio effectively decreases the amplitude of the sine wavetemplate.

After the sine wave template that corresponds to the patient'sinspiratory flow is determined and corrected, if necessary, the volumeof the corrected sine wave template is calculated using any conventionaltechnique. In an analog computation, this is accomplished by integratingover the corrected sine wave template from the start point to the endpoint. In a digital process, this is accomplished by summing the flowsfrom the start point to the end point and dividing by the number ofsummations in this process.

FIG. 19A illustrates an exemplary patient inspiratory waveform 410 and asine wave template 412 determined as discussed above. It can beappreciated from reviewing this figure that there remains a relativelylarge degree of offset between patient inspiratory waveform 410 and asine wave template 412. The present invention accounts for this offsetby effectively shifting the sine wave template, as indicated by arrow414, to overlie the patient inspiratory waveform.

In a preferred embodiment of the present invention, shifting thetemplate to overlie the patient inspiratory waveform is accomplished bydetermining a center C of the patient inspiratory waveform and usingthis center as a new center for the sine wave template. Center C ofpatient inspiratory waveform 410 is determined by finding the points 416and 418 on the inspiratory waveform that corresponds to the FFB value.Finding the points 416 and 418 on the inspiratory waveform thatcorresponds to the FFB value is accomplished by searching up or downfrom the known landmark points 366 and 368, which correspond to the sinebase value (½ FFB). This search is indicated by arrows 420 and 422. Oncethe FFB points are located on inspiratory waveform 410, the center C ofthe inspiratory waveform is taken as ½ the distance between these FFBpoints (416 and 418). Now that center C of inspiratory waveform islocated, the location points defining sine wave template 412 can berecalculated about this center.

Referring now to FIG. 20, a flatness level is calculated by determiningthe volume of the inspiratory waveform 410 above the flatness flatbaseline (FFB) level between the 20% volume point and the 80% volumepoint. Preferably, a weighting constant is applied to this result tomake it less sensitive to slight changes in the shape of the inspiratorywaveform.

In a digital processor, flatness can be determined as follows:

$\begin{matrix}{{Flatness} = {\frac{4*100*{\sum\limits_{20\%\mspace{14mu}{Volume}}^{80\%\mspace{14mu}{Volume}}{{abs}\left( {{Q_{p}(t)} - {{Flatness}\mspace{14mu}{Flat}\mspace{14mu}{Baseline}}} \right)}}}{T_{{20\%} - {80\%}}*{Flatness}\mspace{14mu}{Baseline}}.}} & (4)\end{matrix}$In this relation, the constant value 4 is the weighting constant thatmakes this determination less sensitive to changes in the shape of theinspiratory waveform. Constant value 100 is selected so that theflatness value is expressed as a percentage. Interestingly, the flatnessvalue is large when the inspiratory waveform is sinusoidal and could bezero if the inspiratory waveform is perfectly flat.

Referring now to FIG. 21, roundness is calculated as the differencebetween a patient inspiratory waveform 410 and the sine wave template412 determined as discussed above between the 20% volume point and the80% volume point. This difference is shown in FIG. 21 as shaded areas430. A weighting constant is preferably also applied to the roundnessdetermination to make it less sensitive to slight changes in the shapeof the inspiratory waveform.

In a digital processor, roundness can be determined as follows:

$\begin{matrix}{{{Roundness} = \frac{2*100*{\sum\limits_{20\%\mspace{14mu}{Volume}}^{80\%\mspace{14mu}{Volume}}{{abs}\left( {{{Flow}\mspace{14mu}{{Sine}(t)}} - {Q_{p}(t)}} \right)}}}{{Sine}\mspace{14mu}{Volume}}},} & (5)\end{matrix}$Interestingly, the roundness value is large when the inspiratorywaveform is flat and could be zero if the inspiratory waveform is aperfect sinusoid.

Referring now to FIG. 22, skewness is calculated by first segmenting aninspiratory waveform 432 into regions 434, 436 and 438. Each regioncorresponds to ⅓ of the duration of the inspiratory waveform. Apredetermined amount of the top flows in each region is averaged. Forexample, in a preferred embodiment of the present invention, the top 5%of the flow in each region is averaged. A skewness number for theinspiratory waveform is calculated as the 5% of the middle region 436divided by the 5% of the left region. Stated another way, the skewnessnumber is calculated as:

$\begin{matrix}{{{Skewness}\mspace{14mu}{Number}} = {\frac{{Middle}\mspace{14mu}{Region}\mspace{14mu} 5\%}{{Left}\mspace{14mu}{Region}\mspace{14mu} 5\%}.}} & (6)\end{matrix}$It can be appreciated that the specific manner in which the inspiratorywaveform is segmented, and the percentage of flow from each that areanalyzed to determine the skewness value are subject to variation.

2. Auto-Titration Detection Module

Auto-titration detection module 350 performs two types of trend analysison each of the monitored breath parameters, i.e., weighted peak flow,flatness, roundness, and skew data collected over any period of time,which is typically 2.5 to 20 minutes. The first is a long-term trendanalysis, and the second is referred to as a short-term trend analysis.However, each type of trend analysis requires first collecting the datafor the analysis. Naturally, as more data is input into the trendanalysis, the more likely the analysis will be representative of thepatient's response.

As shown in FIG. 23, the breath parameter data for a patient's breathingcycles 440 are grouped into sets, with each set containing the dataassociated with multiple breathing cycles. In a presently preferredembodiment, each set includes respiratory parameter data for fourbreathing cycles.

The respiratory or breath parameters, i.e., weighted peak flowQ_(Wpeak), roundness, flatness, and skewness, for each breath arecalculated as discussed above. The weighted peak flow data for fourbreaths, for example, are averaged and used to determine a single pointvalue for use in the trend analysis. This same process is conducted forthe other respiratory parameters of roundness, flatness, and skewness.The result is an accumulation of data, as indicated by chart 442, thatis used for trend analysis purposes.

FIG. 24 illustrates an exemplary trend analysis chart, where each pointrepresents the averaged respiratory parameter data over four breathingcycles. Trend analysis of this data involves determining a least squaresfit line, also referred to as a best-fit line, 444 for the data points.It can be appreciated that the slope of best-fit line 444 is indicativeof the degree with which the trend of the data is changing. Next, astandard deviation 446 of the data points about this best-fit line isdetermined over the time interval of interest.

A variety of different types of analysis can be done based on this data.For example, the present invention contemplates determining a percentchange and a difference value of the trend data. The percent change iscalculated as:

$\begin{matrix}{{{\%\mspace{14mu}{change}} = {\frac{{{end}\mspace{14mu}{point}} - {{start}\mspace{14mu}{point}}}{mean} \times 100}},} & \left( {7a} \right)\end{matrix}$where the end point is a point on best-fit line 444 near the end of thecollected data, such as point 448, the start point is a point onbest-fit line 444 near the start of the collected data, such as point450, and the mean is the mean value of the data points between the startand end points. An equivalent calculation for the percent change can beexpressed as:

$\begin{matrix}{{{\%\mspace{14mu}{change}} = {\frac{{slope} \times {trend}\mspace{14mu}{length}}{mean} \times 100}},} & \left( {7b} \right)\end{matrix}$where slope is the slope of the best-fit line 444 and the trend lengthis the length of the trend, indicated as the time between the startpoint and the end point.

The difference value is calculated as the difference between the valueof the end point and the start point expressed as:difference value=end point−start point.  (7c)The equivalent representation of this equation can be expressed as:difference value=slope×trend length.  (7d)

According to a preferred embodiment of the present invention, whenanalyzing the weighted peak flow data, only the percent change is used.When analyzing the roundness, flatness, and skewness data, only thedifference value of the trend data is used because, in a preferredembodiment of the present invention, these raw measures are alreadyrepresented as percentages. An error window, defined by a percent changeor difference as described above, is compared to predeterminedthresholds to determine whether the change in the data, i.e., the trend,has exceeded acceptable levels. It should be noted that the type ofanalysis (percent change or difference) depends on the type of raw dataused in the trends analysis.

As noted above, auto-titration detection module 350 looks at ashort-term trend and a long-term trend of the accumulated datapoints—recall that each data point contains an average of the parameterdata for four breathing cycles. When performing the long-term trendanalysis, the percent change or the difference value (depending on theparameter of interest) is evaluated over time to determine whether thesetrend analysis criteria fall outside predetermined thresholds. Whenperforming the short-term trend, each newly collected data point iscompared to the data points already collected in an effort to locateanomalies in the monitored parameters relative to the trended data.

a. Long-Term Trend

To perform the long-term trend analysis, the best-fit line for thetrended data, which has an associated standard deviation for the datapoints around that line, is used to determine a trend error window. Thetrend error window represents a range of error for the trend data. Thetrend error window is a function of the standard deviation for thatbest-fit line, the number of data points used in the trend calculation,and the desired confidence level, and is determined using anyconventional technique, such as using a look-up table, once the inputcriteria (standard deviation, # of samples (data points), and confidencelevel) are established.

In the present invention, the confidence level used in selecting thetrend error window is determined based on an empirical evaluation of thedata. It was determined from this empirical analysis that, for purposesof the present invention, an 80% confidence level is appropriate for thetrend error window. However, those skilled in the art can appreciatethat this level can be varied and still provide meaningful results. Inessence, in selecting an 80% confidence level, the present inventionseeks to say, with an 80% level of confidence, that the best-fit line,with its associated scatter of data, represents the true trend of thedata being analyzed.

Once a trend error window is determined, this range of error isconverted into an error window based on the difference value or thepercent change discussed above. This can be accomplished by applying thecalculations discussed in equations (7b) and (7d) to the trend errorwindow. In this case, the slope of the best-fit line would berepresented by a range of slopes that take into account the best-fitline 444 and its associated trend error. Once the error window isconverted to a difference or percent change, it is provided fromauto-titration detector 350 to auto-titration monitor 352, which usesthis trend based information, as discussed below, to judge the patient'sresponse changes to the delivered pressure.

b. Short-Term Trend

The short-term trend analysis attempts to distinguish relatively quickpatient response to the delivered pressure. Therefore, rather thanlooking at the changes in the trend data over time, the short-term trendanalysis function of auto-titration detection module 350 in combinationwith auto-titration monitoring module 352, analyzes each data point asit is generated against two detection criteria. The auto-titrationdetection module establishes the short-term trend criteria, and theauto-titration monitoring module 350 analyzes the newly generated datapoint against these criteria.

The first short-term trend criteria determined by the auto-titrationdetection module is a prediction interval. The goal of the predictioninterval is to provide a range of values against which the newlygenerated data point is compared. The prediction interval is determined,using standard statistical analysis techniques, based on the standarddeviation of the data points about the best-fit line, the number ofsamples or data points in the trend analysis calculation, and thedesired confidence level. In the present invention, the confidence levelused to select the prediction interval is determined based on anempirical evaluation of the data. It was determined from this empiricalanalysis that, for purposes of the short-term trend analysis, a 95%confidence level is appropriate. However, those skilled in the art canappreciate that this level can be varied and still provide meaningfulresults. Based on these criteria, the prediction interval represents arange of values in which we are 95% confident that the next generateddata point will fall within this range of values.

The second short-term trend criteria determined by the auto-titrationdetection module is simply a “start of trend data point,” which is adata point on the best-fit line at the start of the collection of data.The start of trend data point is similar to data point 450 in FIG. 24.As previously described for the long-term trend, a percent change anddifference is calculated for the short-term. This is accomplished byusing equations (7a) and (7c) described above. For the short-termcalculation, the end point is defined as the value of the current datapoint, and the start point is defined as the start of trend point,similar to data point 450 in FIG. 24. As discussed below, the predictioninterval and the short-term percent change (or difference, i.e.,dependent upon the individual breath measure, consistent with thatdescribed for the long-term trend) are provided from auto-titrationdetection module to auto-titration monitoring module 352.

3. Auto-Titration Monitoring Module

Auto-titration monitoring module 352 uses the trend information providedby auto-titration detection module 350 in a voting process to determinethe patient's response to a pressure being delivered to the airway. Forexample, the auto-titration monitor determines whether or not theprofile of the patient flow waveform is improving or degrading, thusindicating whether airway flow restriction may be improving ordegrading.

a. Long-Term Trend Voting

FIG. 25 is a chart 459 explaining, by illustration, the voting conductedon the information provided by the long-term trend analysis. At thecenter of chart 459 is a voting window 461 that is bounded by an upperthreshold 462 and a lower threshold 464. There are three levels ofvoting in this chart: 1=getting better, 0=no change, −1=getting worse.

The trended data, along with its associated statistical error, whichcorresponds to an error window 464 calculated during the long-term trendanalysis performed by the auto-titration detector, is compared tothresholds 460 and 462. In order to produce a vote of 1, the entireerror window must exceed an assigned threshold level. This thresholdlevel varies from measure to measure, but typically ranges from 7% to8%. In FIG. 25 the 8% value is selected. If the entire error band 464 isabove threshold level 460, a vote of 1 is generated, as indicated byregion 466. Similarly, if the entire error band 464 is below thresholdlevel 462, as indicated by region 468, a vote of −1 is generated.Otherwise a vote of zero (0) is generated, region 470.

b. Short-Term Trend Voting

The short-term trend analysis described above and the short-term votingscheme described below is designed to detect short-term or relativelysudden changes in the patient's flow profile. This is accomplished bycomparing a single grouping of breaths (i.e., one data point, whichcontains 4 breaths) to the first and second short-term trend criteriadiscussed above and to determine whether that group has shown astatistically significant change with regard to the long-term trendeddata.

If (1) the newly generated data point is equal to or outside theprediction interval and (2) the data point differs from the start oftrend data point by a predetermined threshold amount, the data point(i.e., breath group) is deemed to represent a significant change withrespect to the beginning of the long-term trend. Therefore, if both ofthese conditions are met, the short-term trend generates a vote of 1 or−1, depending on whether the data point is above or below the start oftrend data point. Otherwise, a vote of zero (0) is generated. Thethreshold for the percent change or difference between the data pointand the start of trend data point used for short-term trending variesfrom measure to measure, but typically ranges from 9% to 14%.

c. Final Voting

Once a long-term vote and a short-term vote has been issued for eachindividual breath measure, the votes from all the measures are thenaccumulated into a single, final vote. The following table summarizesthe final voting process:

Long-Term Vote Short-Term Vote Result Q_(Wpeak) (−1, 0, 1) (−1, 0, 1) aRoundness (−1, 0, 1) (−1, 0, 1) b Flatness (−1, 0, 1) (−1, 0, 1) cSkewness (−1, 0, 1) (−1, 0, 1) d Final Vote x = a + b + c* + d

The value placed in the “Result” column for each breath parameter is thevalue of the long-term vote, unless the long-term vote is zero. If thelong-term vote is zero, the short-term vote value is placed in theresults column for that breath parameter. The results are summed togenerate the final vote.

The only other caveat implemented by the present invention is that theflatness breath parameter is ignored when summing for the final vote ifthe flatness result is non-zero and if it is inconsistent with the othernon-zero voting breath parameters associated with the shape of theinspiratory waveform, i.e., roundness and skewness. This is why anasterisk is placed next to “c” in the above table, meaning that incertain situations the flatness value “c” is ignored. For example, theresult for flatness is 1, and either the roundness or the skewnessparameter is a −1, the flatness result is ignored in the summation forthe final vote. Similarly, if the result for flatness is −1, and theeither the roundness or the skewness parameter is a 1, the flatnessresult is ignored in the summation for the final vote.

The final vote “x” from the above table can have a range of −4 to 4 andis used to determine the three primary conditions about the profile ofthe patient flow waveform. The condition of the patient's inspiratoryflow is also indicative of the patient's response to the pressure beingprovided to the airway. The three primary conditions that summarize apatient's response to the pressure, and the final vote value associatedwith each condition, are given below:

1) statistically significant degradation, x≦−2

2) statistically no change, and −2<x<2

3) statistically significant improvement. x≧2

All three of these conditions can be determined independent of whetherthe auto-titration controller is increasing, decreasing, or holding theIPAP, EPAP, or both constant. The following table summarizes where eachcondition (1), (2) or (3) falls for each value of x:

x = −4 x = −3 x = −2 x = −1 x = 0 x = 1 x = 2 x = 3 x = 4 (1) (1) (1)(2) (2) (2) (3) (3) (3)

As discussed in greater detail below, during certain pressure controloperations performed by auto-titration controller 354, a fourthcondition, which is interposed between conditions (2) and (3), is added.This additional condition, which is designated as condition (2.5)because it is between conditions (2) and (3), corresponds to the patientcondition, i.e., the patient inspiratory waveform, exhibitingstatistically marginal improvement. This condition is deemed to occur ifthe final vote during certain pressure control operations equals +1,i.e., x=+1. The four conditions that summarize a patient's response tothe pressure, and the final vote value “x” associated with eachcondition, are given below for this situation:

1) statistically significant degradation, x≦−2

2) statistically no change, −2<x<1

2.5) statistically marginal improvement, and x=1

3) statistically significant improvement. x≧2

The following table summarizes where each condition (1), (2), (2.5) or(3) falls for each value of x in this situation:

x = −4 x = −3 x = −2 x = −1 x = 0 x = 1 x = 2 x = 3 x = 4 (1) (1) (1)(2) (2) (2.5) (3) (3) (3)It is to be understood that greater or fewer conditions can be provideddepending on how fine tuned the auto-titration control layer should beto changes in the patient's condition.

4. Auto-Titration Control Module

The auto-titration controller uses the final voting level describedabove, which is an indication of the patient's response to the pressurebeing provided to his or her airway by the pressure support system,along with its current mode of operation, to determine what actions totake. Three general cases are presented below to describe the behaviorof the auto-titration controller.

a. Case 1—Startup

FIG. 26 illustrates an IPAP curve 500 output by the pressure supportsystem during Case 1. That is, in this embodiment, the IPAP is adjustedwhile the EPAP is held constant. The EPAP will only change if necessaryto maintain the PS_(min) level or PS_(max) level. When the pressuresupport system is turned on, it will enter a hold period 502 and collectdata. In a preferred embodiment, this hold period lasts 5 minutes.However, the duration of the hold period can be a value other than 5minutes, so long as enough time elapses to collect a meaningful amountof data. At the end of this period, auto-titration controller 354initiates a recovery state in which the patient IPAP is ramped upslowly, with a targeted increase of 2.0 cmH₂O, and at a rate of increaseof approximately 0.5 cmH₂O/min.

During this ramping, the trend data is continually examined byauto-titration monitor 352 using the four conditions, (1), (2), (2.5)and (3), to determine if the patient flow profile has experiencedstatistically significantly degradation—condition (1), statistically nochange—condition (2), statistically margin improvement—condition (2.5),or statistically significant improvement—condition (3) during the rampperiod. However, no action is taken on this determination until ˜2.5minutes have elapsed since the start of the pressure increase. This 2.5minute lockout window 504 is provided to allow the system to collectenough data for trending purposes. It can be appreciated that theduration of the lockout interval can vary, for example, between 2-4minutes. However, the longer this lockout window, the less responsivethe system will be to treat any potential breathing disorders.

If the patient's inspiratory flow waveform has improved or degradedduring ramp 506, the ramping and trending continues until theimprovement or degradation ceases, for example the patient's conditionchanges from (3) to (2.5) or the patient's condition changes from (1) to(2). For the case where the patient condition changes from (3) to (2.5),the auto-titration controller 354 will decrease IPAP by some smallamount, typically 0.5 cmH2O, and then a 5-minute hold period will bestarted, as indicated by pressure curve 508. If there is no improvementduring the ramp, i.e., the patient's inspiratory flow profile stays thesame—condition (2) or condition (2.5), auto-titration controller 354decreases the IPAP 2.0 cmH2O, as indicated by pressure curve 510, and a5 minute hold period 512 is then started. This sequence of pressurecontrol is intended to determine if flow limitation exists in thewaveforms, and to locate an ideal pressure at which flow limitation nolonger exists. If flow limitation is detected during any hold period(indicating that the patient may have changed position or sleep stage),the slow ramp up will again be initiated.

b. Case 2—Return from a Higher Priority Controller

During the course of the pressure support therapy, which typicallyrepeats throughout the night, higher level controllers, such as snorecontroller 144 or apnea/hypopnea controller 168, may temporarily takecontrol and perform pressure changes as discussed above. Once all activehigh priority controllers are finished, control is returned toauto-titration controller 354. Upon receiving control from a higherpriority controller, the auto-titration controller performs the sameactions as described in Case 1 above, with the exception that theinitial 5 minute hold period is replaced by a ˜3 to 3.5 minute period.

c. Case 3—Patient Pressure Decreases

When the last 5-minute hold period from either Case 1 or Case 2 iscompleted, as indicated by pressure curve 520 in FIGS. 27A and 27B, thenext search sequence is started. In this search sequence, the IPAPdelivered by the system is slowly lowered at a rate of 0.5 cmH₂O/minute,as indicated by curve 522. Again, in this exemplary embodiment, EPAP isheld constant so long as the PS_(min) and PS_(max) are not exceeded.Prior to starting the decrease in pressure, the breath measure trendsare initialized with up to the last three minutes of available data.

After ramping IPAP down 0.5 cmH₂O, the trend data is then continuallyexamined to determine if the patient inspiratory flow profile hasdegraded or not over the ramp period. In this trend analysis, only thethree patient conditions (1), (2) and (3) are taken into consideration.If there is no patient flow profile degradation detected (condition(2)), the IPAP ramp and trending will continue until a minimum systempressure P_(min) (which is either IPAP_(min) or EPAP_(min)) is reached,as shown in FIG. 27A. Thereafter, auto-titration controller 354 beginsthe Case 1 IPAP control discussed above and begins a 5 minute holdperiod 502.

If, during the IPAP pressure decrease, the patient inspiratory flowprofile has degraded, for example, moved from condition (2) to condition(1), the IPAP will be quickly increased 1.5 cmH₂O, curve 526, and thenheld constant for up to 10 minutes, curve 528. See FIG. 27B. Once the 10minute hold period ends, auto-titration controller 354 directly entersthe recovery state discussed above with respect to Case 1, and initiatesan IPAP increase 506.

This entire sequence is intended to determine the pressure at which flowlimitation occurs and then raise the pressure to an ideal setting. Thissequence is repeated throughout the night to locate the optimal IPAP aspatient conditions change and to improve comfort by keeping the pressureas low as practical. If flow limitation is detected during any holdperiod (indicating that the patient may have changed position or sleepstage), the IPAP slow ramp up (recovery state) will again be initiated.

During this IPAP decrease, where the auto-titration controller issearching for a potential flow limitation point, the chance of a snoreoccurring is increased. For this reason, the present inventioncontemplates reducing the required number of snore events from three totwo that will cause snore monitoring module 142 to request that thesnore controller take control. This effectively increases the system'ssensitivity to snore during the pressure decrease interval.

During any hold period, such as hold period 502, 508, 512, 520, or 528,auto-titration controller 354 can enter the recovery state discussedabove in Case 1 to attempt to provide the optimal IPAP and EPAP to thepatient. This may occur, for example, if the trends data analyzed duringthe hold indicated that the patient's inspiratory waveform profile isexperiencing a statistically significant degradation (condition (1)).

The present invention has been described above as adjusting only theIPAP and leaving the EPAP alone, unless necessary to maintain PS_(min)or PS_(max). It is to be understood that this is an exemplary embodimentof the present invention. The present invention also contemplatesadjusting both the IPAP and the EPAP during the auto-titration process.For example, both the IPAP and EPAP can be adjusted so as to maintainthe PS constant at all times.

J. Detection of Central Versus Obstructive Apnea/Hypopnea Events

In Section G above, in which the operation of the apnea/hypopnea controllayer is discussed, it was noted that A/H detection module 164 cannotdetect the difference between obstructive apnea/hypopnea events andcentral apnea/hypopnea events but compensates for this shortcoming bythe manner in which the pressure is delivered to the patient. However, afurther embodiment of the present invention contemplates detecting thedifference between obstructive apnea/hypopnea events and centralapnea/hypopnea events A/H via detection module 164. This isaccomplished, for example, by monitoring the patient's inspiratorywaveform during the apnea/hypopnea period, immediately after the end ofthe apnea/hypopnea period, or during both these periods as discussedbelow.

If it is determined that the patient is experiencing an obstructiveapnea/hypopnea event, the IPAP and EPAP are delivered to the patient asdiscussed above in Section G. If, however, the patient is experiencing acentral apnea/hypopnea event, it is preferable not to increase the IPAPor EPAP. It is generally recognized that increasing the IPAP and/or EPAPdelivered to the patient does not treat an episode of centralapnea/hypopnea. Therefore, the present invention contemplatesmaintaining the IPAP and EPAP delivered to the patient at the currentlevel or even decreasing the IPAP or EPAP if the patient is deemed to beexperiencing a central apnea/hypopnea.

Maintaining the IPAP and EPAP at its current level is accomplished,according to one embodiment of the present invention, by causing the A/Hdetection module to reject the apnea/hypopnea event as an apnea/hypopneaevent if it is determined to be a central apnea/hypopnea event. In whichcase, the system acts as if no apnea/hypopnea event was detected anddoes not request that A/H controller 168 take control of the system. Thepresent invention also contemplates reducing the IPAP and/or EPAPdelivered to the patient if it is determined that the patient isexperiencing a central apnea.

The manner in which the present invention discriminates betweenobstructive/restrictive apnea/hypopnea events and central apnea/hypopneaevents is discussed below with reference to FIGS. 28-30, whichillustrate exemplary patient flow waveforms during anobstructive/restrictive apnea/hypopnea events (FIGS. 28 and 30) andduring a central apnea/hypopnea event (FIG. 29). The determination ofwhether the patient is experiencing an obstructive/restrictiveapnea/hypopnea event or a central apnea/hypopnea event is preferablymade by A/H detection module 164, which supplies its determination toA/H monitoring module 166 to actuate A/H controller 168 so that theappropriate pressure control can be made as discussed above.

In a presently preferred exemplary embodiment, the patient's inspiratorywaveform during the apnea/hypopnea period is monitored to determinewhether he or she is experiencing an obstructive/restrictiveapnea/hypopnea event or a central apnea/hypopnea event. In thehypothetical patient flow waveforms 600 and 602 in FIGS. 28 and 29,respectively, the apnea/hypopnea event begins at point 604 andterminates at point 606, which is determined as discussed above inSections G(3) and G(4). It should be noted that waveforms 600 and 602are provided to illustrate a technique used by an exemplary embodimentof the present invention to determine whether the patient isexperiencing an obstructive/restrictive apnea/hypopnea event or acentral apnea/hypopnea event. These waveforms may not be to scale andmay not accurately represent an actual patient flow. The dashed lines inFIGS. 28 and 29 illustrate the patient flow valley that occurs during anapnea/hypopnea event. It is in this valley or apnea/hypopnea period thatthe present invention examines the shape of the patient's flow todetermine whether he or she is experiencing an obstructive/restrictiveapnea/hypopnea event or a central apnea/hypopnea event.

More specifically, the present inventors understood that during anobstructive/restrictive apnea/hypopnea event, the shape characteristicsof the patient's inspiratory waveform tends to exhibit the same shapecharacteristics associated with a restricted airflow. Namely, during anobstructive/restrictive apnea/hypopnea event, the waveform exhibits anincrease in flatness (becomes flatter), a decrease in roundness (becomesless round), an increased skewness (becomes more skewed) (as shown inFIG. 22) or any combination of these characteristics.

For example, in FIG. 28 inspiratory waveforms 610 occurring during theapnea/hypopnea period between points 604 and 606 tend to have anincreased degree of flatness, a lack of roundness, an increasedskewness, or any combination of these characteristics, indicating thatwaveform 600 represents an obstructive/restrictive apnea/hypopnea ratherthan a central apnea/hypopnea. In FIG. 29, on the other hand,inspiratory waveforms 612 occurring during the apnea/hypopnea periodbetween points 604 and 606 tend to have no increased degree of flatness,relatively normal roundness, and no increase in skewness, indicatingthat waveform 602 represents a central apnea/hypopnea rather than acentral apnea/hypopnea. Thus, the present invention contemplatesmonitoring the flatness, roundness and skewness of the waveformsoccurring during the apnea/hypopnea period via A/H detection module 164to determine whether the patient is experiencing anobstructive/restrictive apnea/hypopnea event or a central apnea/hypopneaevent. In a presently preferred embodiment, all of these shape criteriaare monitored during the apnea/hypopnea period. It is to be understoodthat the present invention contemplates monitoring as few as onecriteria, such as flatness, to make this determination.

In a second embodiment of the present invention, the patient's airflowwaveform during a period immediately after the end of the apnea/hypopneais monitored to determine whether he or she experienced anobstructive/restrictive apnea/hypopnea event or a central apnea/hypopneaevent. More specifically, the present inventors understood that thepatient's respiratory flow is different at the end of the apnea/hypopneaevent depending on whether the patient suffered anobstructive/restrictive apnea/hypopnea or a central apnea/hypopnea. Morespecifically, as shown in FIG. 30, which depicts a patient's respiratoryflow waveform 620 during an obstructive/restrictive apnea/hypopneaevent, it has been determined that at the termination of an obstructiveapnea/hypopnea event, a patient often tends to take a relatively largegasping breath or series of gasping breaths, generally indicated asbreaths 622 in FIG. 30. At the end of a central apnea/hypopnea event, onthe other hand, the patient does not tend to take large breaths. SeeFIG. 29.

Thus, the present invention contemplates determining whether the patienthas experienced an obstructive/restrictive apnea/hypopnea event or acentral apnea/hypopnea event by determining whether the patient hastaken large gasping breaths at the end of the apnea/hypopnea. This isaccomplished, for example, by the tidal volume of the breathsimmediately following the end of the apnea/hypopnea period and comparingthis volume against a predetermined threshold volume. If the breathshave a tidal volume that exceeds the threshold level, the patient isdeemed to have experienced an obstructive/restrictive apnea/hypopnea. Inwhich case, the pressure is delivered to the patient as discussed abovein Section G.

It should be noted that the present invention contemplates monitoringrespiratory characteristics other than tidal volume in order todetermine whether the patient is taking large, gasping breaths at theend of the apnea/hypopnea period. For example, the peak flow can also bemeasured against a threshold to evaluate whether the patient is takingrelatively large breaths.

Two techniques have been discussed above for determining whether apatient is experiencing an obstructive/restrictive apnea/hypopnea eventor a central apnea/hypopnea event. These techniques can be used alone orin combination to make this determination. Furthermore, the presentinvention also contemplates using any conventional technique fordetecting a central apnea, either alone or in combination with the twotechniques discussed above, such as monitoring for cardiogenicrespiratory events or testing the airway for patency during anapnea/hypopnea period.

In a presently preferred embodiment, the A/H control layer does notdiscriminate between obstructive/restrictive and central apnea/hypopneaevent unless the IPAP or EPAP being delivered to the patient is above acertain threshold. This threshold ensures that a pressure treatment isprovided if the patient is being treated with a relatively low IPAP orEPAP regardless of whether the apnea/hypopnea was central orobstructive. If the IPAP or EPAP is below this threshold, the systemperforms the pressure treatment as discussed above in Section G. If,however, the patient is being treated with a relatively high IPAP orEPAP, i.e., an IPAP or EPAP above the pressure threshold, it ispreferable to determine whether the apnea/hypopnea is central orobstructive, because, as noted above, increasing the pressure for acentral apnea provided no therapeutic effect.

In a preferred embodiment, the pressure threshold is set at 8 cmH₂O,which has been determined from analysis of clinical data to be apressure level that provides a moderate degree of pressure support formost patients, but is not too high as to cause unduly high pressures tobe delivered should the patient be experiencing a centralapnea/hypopnea. It is to be understood that this threshold can haveother values and can be adjustable depending on the characteristics ofthe patient or the patient's history.

K. Conclusion

It can be appreciated that the present invention contemplates providingadditional control layers to those shown in FIG. 2. Likewise one or moreof the control layers shown in FIG. 2 can be deleted depending on thedesired operating capability of the pressure support system.Furthermore, the present invention is not intended to be limited to theprioritization hierarchy shown in FIG. 2. For example, theapnea/hypopnea control layer (priority #6) can be given a higherpriority by interchanging it with the big leak control layer (priority#5).

With reference to FIG. 2, request processor 106 resets detection modules102, monitoring modules 104, and control modules 100 generally based onchanges between control modules. Detection modules 102 are generallyonly reset by machine based control layers above line 108. Monitoringmodules 104 are generally reset after a control layer completes itspressure treatment and has given control of the pressure support systemback to the lower control layers. This is done so that the monitors cankeep track of the patient's progress since the last pressure treatment.This is also important in order to avoid over-treating the patient in asituation where two overlapping patient events occur, e.g., hypopneawith snoring. If the snore controller is actively treating the snoringcondition, and, thus, is indirectly aiding in the treatment of thesimultaneously occurring hypopnea, the hypopnea monitor will be reset,thus, inhibiting an additional follow-on request from the hypopneamonitor. Control modules 100 are reset based on the priority of thecurrent control layer. When the current controller gives control of thepressure support system back to the lower control layers, generally alllower control layers are reset so that their processing will start overfrom where the last control layer left off.

L. Alternative Applications

In the embodiments described above, the auto-titration function of thepresent invention is applied to a bi-level pressure support system,i.e., a system in which the pressure delivered to the patient isgenerally higher during inspiration and than during expiration. It is tobe understood, that the present invention contemplates using thebi-level auto-titration techniques described above in combination withother modes of pressure support. One such mode suitable for use with theauto-titration technique of this invention includes, but is not limitedto a Bi-Flex® therapy. In other words, the auto-titration technique ofthe present invention can be applied to control a base or target IPAPlevel or EPAP level and other pressure control techniques can be appliedin combination with these targets.

U.S. Pat. Nos. 5,535,738; 5,794,615; 6,105,575; 6,609,517, and6,932,084, the contents of which are incorporated herein by reference,describe a pressure support technique referred to as proportionalpositive airway pressure (PPAP), in which a base pressure, such as IPAPor EPAP, is modified to produce a pressure support therapy for thepatient. That is, the present invention contemplates using theauto-titrating techniques discussed above to control the base pressuretaught by these PPAP patents.

In one embodiment of the PPAP pressure support technique, referred to asBi-Flex pressure support, a first base pressure is the IPAP that isapplied to the patient during the inspiratory phases of the respiratorycycle, and a second base pressure is the EPAP that is applied to thepatient during the expiratory phases. In one embodiment, during at leasta portion of the expiratory phase, the EPAP is reduced or modified, inwhole or in part, by some amount to provide a degree of pressure reliefduring at least a portion of the expiratory phase. In anotherembodiment, which can be used alone or in combination with theexpiratory pressure relief embodiment, the IPAP is reduced or modified,in whole or in part, by some amount to provide a modified inspiratorypressure curve or inspiratory pressure relief.

A further embodiment of the present invention contemplates using onlythe event monitoring and detection capabilities of the auto-titrationtechniques discussed herein, with no additional control based onsearching or in response to events. In essence, this embodimentcorresponds to enabling detection module 102 and monitoring module 104while disabling control module 100 and request processor 106. Thisvariation of the present invention provides the ability to monitor thepatient, the operation of the pressure support system, or both.

The present invention also contemplates allowing the user to set therise time, the shape of the rise, the slope, or any other feature of thepressure increase that occurs when the system transitions from thepressure delivered during expiration to the pressure delivered duringinspiration, e.g., from EPAP to IPAP. Similarly, the present inventioncontemplates allowing the user to set the fall time, the shape of thefall, the slope, or any other feature of the pressure increase thatoccurs when the system transitions from the pressure delivered duringinspiration to the pressure delivered during expiration, e.g., from IPAPto EPAP.

It can be appreciated that there are instances, such as in the treatmentof snoring, where either the IPAP, EPAP or both can be adjusted to treatthe patient. The choice of which pressure control may be open to debatein the medical community or may be specific to that patient, i.e., onepatient may be treated best with an IPAP change, while another may betreated best with an EPAP change. For these reasons, the presentinvention contemplates that the decision as to whether to adjust IPAP orEPAP can be made based on certain criteria and/or selected by the user.For example, the controller can monitor the condition of the patient,the treatment provided to the patient, or both and select either theIPAP or the EPAP for control. In an exemplary embodiment, the systemmonitors the pressure support level of the current treatment pressureand the controller chooses between adjusting either the EPAP or the IPAPbased on the monitored condition

The system can also be set up such that the user or the caregiver canset which one of the IPAP, EPAP, or both is adjusted based on amonitored event. For example, some physicians may prefer to treatsnoring by adjusting EPAP, while others may prefer to adjust IPAP, whilestill others may prefer to adjust some combination of IPAP and EPAP. Aninput to the system can be provided to allow the physician or othercaregiver to select which one of these is controlled if snoring isdetected. The present invention also contemplates that the controllercan be programmed to treat the patient by controlling the IPAP and, ifsnoring still occurs, switch to controlling the EPAP, or vice versa. Itcan be appreciated that a wide variety of possible scenarios exist formonitoring the condition of the patient, the condition of the pressuresupport system (such as the treatment being provided), an input providedto the pressure support system, or any combination thereof and forselecting whether to control the IPAP or the EPAP based on the monitoredcriteria.

The present invention also contemplates automatically controlling therise time, the fall time or both, for example to optimize patientcomfort. A description of a system that automatically adjusts rise time,fall time, or both is disclosed in U.S. Pat. Nos. 6,532,960 and6,640,806, the contents of which are incorporated herein by reference.

Event monitoring and detection, in the absence of any auto-titratingpressure control device, can be applied to any existing mode of pressuresupport therapy, including, but not limited to CPAP, BiPAP, C-Flex, andBi-Flex therapies. Detected events can be logged internally, provided toa removable medium, transmitted from the pressure support device (suchas serially, wireless, etc.), or any combination thereof. Thisinformation is useful, for example, in determining whether the patientreceived adequate therapy based on the detected events.

It can be appreciated that the present invention describes a system andmethod similar to the Auto-CPAP techniques discussed in the parentapplications, that may help increase compliance in CPAP intolerantpatients by using the bi-level mode of positive airway pressure therapy,which is also referred to as an auto-bi-level mode. Auto-bi-level, asdescribed herein, stabilizes the airway with a baseline EPAP level setto prevent airway occlusion and snoring and an IPAP level set to preventhypopneas and flow limitation. In addition to the benefits described forauto-CPAP, auto-bi-level is expected to provide further comfort for CPAPintolerant patients by allowing separate pressure levels for inspirationand expiration.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims.

What is claimed is:
 1. A bi-level auto-titration pressure support systemcomprising: (a) a pressure generating system adapted to generate a flowof breathing gas at an inspiratory positive airway pressure (IPAP)during inspiration and at an expiratory positive airway pressure (EPAP)during expiration; (b) a patient circuit having a first end adapted tobe coupled to the pressure generating system and a second end adapted tobe coupled to an airway of a patient; (c) a monitoring system associatedwith the patient circuit or the pressure generating system and adaptedto measure a parameter indicative of a pressure at a patient's airway, aflow of gas in such a patient's airway, or both and to output a pressuresignal, a flow signal indicative thereof, respectively, or both; and (d)a controller coupled to the monitoring system and the pressuregenerating system, for controlling the pressure generating system basedon the output of the monitoring system, wherein the controller isprogrammed to monitor each of the following conditions: (1) snoring, (2)apneas, (3) hypopneas, and (4) a big leak in the pressure supportsystem, wherein the big leak is a system leak that is substantiallygreater than any intentional system leaks, and wherein the controller isprogrammed to: (a) increase the EPAP without changing the IPAP,responsive to a determination that the patient is experiencing onlyapneas and not hypopneas, or is experiencing a combination of apneas andhypopneas, (b) decrease the IPAP without changing the EPAP, responsiveto a detection of the big leak, (c) increase the IPAP without changingthe EPAP, responsive to a determination that the patient is experiencingonly hypopneas and not apneas, and (d) either increase the EPAP withoutchanging the IPAP increase the IPAP without changing the EPAP responsiveto a determination that the patient is snoring.
 2. The system of claim1, wherein the controller performs an auto-titration process in whichthe controller adjusts the IPAP and the EPAP to determine an optimalIPAP for the patient.
 3. The system of claim 1, wherein the controller:(a) causes the pressure generating system to reduce or modify the EPAPduring at least a portion of the expiratory phase by some amount toprovide a degree of pressure relief during the at least a portion of theexpiratory phase; (b) causes the pressure generating system to reduce ormodify the IPAP during at least a portion of the inspiratory phase bysome amount to provide a degree of pressure relief during the at least aportion of the inspiratory phase; or (c) causes the pressure generatingsystem to reduce or modify the EPAP during at least a portion of theexpiratory phase by some amount to provide a degree of pressure reliefduring the at least a portion of the expiratory phase, and causes thepressure generating system to reduce or modify the IPAP during at leasta portion of the inspiratory phase by some amount to provide a degree ofpressure relief during the at least a portion of the inspiratory phase.4. The system of claim 1, further comprising an input device forselecting activation of a ramp cycle, wherein the controller (a)decreases the IPAP and the EPAP responsive to activation of the pressureramp, and (b) thereafter increases the IPAP and EPAP, and wherein thecontroller adjusts the IPAP and the EPAP during a pressure increaseportion of the ramp cycle.
 5. A method of providing a bi-levelauto-titration pressure support therapy to a patient, comprising: (a)generating a flow of gas at an inspiratory positive airway pressure(IPAP) during inspiration and at an expiratory positive airway pressure(EPAP) during expirations; (b) coupling a first of a patient circuit tothe pressure generating system and coupling a second end of the patientcircuit an airway of a patient; (c) monitor each of the followingconditions: (1) snoring, (2) apneas, (3) hypopneas, and (4) a big leakin the pressure support system, based on a pressure of a rate of theflow of gas, wherein the big leak is a leak that is substantiallygreater than any intentional system leaks; and (d) (i) increasing theEPAP without changing the IPAP responsive to determining that thepatient is experiencing only apneas and not hypopneas, or isexperiencing a combination of apneas and hypopneas, (ii) decreasing theIPAP without changing the EPAP responsive to detecting the big leak,(iii) increasing the IPAP without changing the EPAP responsive todetermining that the patient is experiencing only hypopneas and notapneas, and (iv) either increasing the EPAP without changing the IPAP orincreasing the IPAP without changing the EPAP responsive to determiningthat the patient is snoring.
 6. The method of claim 5, furthercomprising performing an auto-titration process in which the IPAP andthe EPAP are adjusted to determine an optimal IPAP for the patient. 7.The method of claim 5, further comprising: (a) reducing or modifying theEPAP during at least a portion of the expiratory phase by some amount toprovide a degree of pressure relief during the at least a portion of theexpiratory phase; (b) reducing or modifying the IPAP during at least aportion of the inspiratory phase by some amount to provide a degree ofpressure relief during the at least a portion of the inspiratory phase;or (c) reducing or modifying the EPAP during at least a portion of theexpiratory phase by some amount to provide a degree of pressure reliefduring the at least a portion of the expiratory phase, and reducing ormodifying the IPAP during at least a portion of the inspiratory phase bysome amount to provide a degree of pressure relief during the at least aportion of the inspiratory phase.
 8. The method of claim 5, furthercomprising choosing between adjusting either the EPAP, the IPAP, or acombination of EPAP and IPAP based on a monitored condition of thepatient, a monitored condition of a pressure support system thatprovides a flow of gas to such a patient, an input provided to thepressure support system, or any combination thereof.
 9. The method ofclaim 5, executing a ramp cycle that includes: (a) decreasing the IPAPand the EPAP responsive to activation of the ramp cycle, and (b)thereafter increasing the IPAP and EPAP, wherein the IPAP and the EPAPare permitted to be adjusted during a pressure increase portion of theramp cycle.
 10. A bi-level auto-titration pressure support systemcomprising: a pressure generating system adapted to generate a flow ofbreathing gas at an inspiratory positive airway pressure (IPAP) duringinspiration and at an expiratory positive airway pressure (EPAP) duringexpiration; a patient circuit having a first end adapted to be coupledto the pressure generating system and a second end adapted to be coupledto an airway of a patient; a monitoring system associated with thepatient circuit or the pressure generating system and adapted to measurea parameter indicative of a pressure at a patient's airway, a flow ofgas in such a patient's airway, or both and to output a pressure signalindicative thereof, a flow signal indicative thereof, respectively, orboth; and a controller coupled to the monitoring system and the pressuregenerating system, and adapted to control the IPAP and the EPAP based onthe output of the monitoring system, wherein the controller isprogrammed to operate according to one control layer in a set ofprioritized control layers, wherein each control layer in the set ofprioritized control layers competes for control of the pressuregenerating system with the other control layers, and wherein eachcontrol layer implements a unique pressure control process forcontrolling the IPAP, the EPAP, or both.
 11. The system of claim 10,wherein each control layer in the set of prioritized control layerincludes: a detection module that receives the pressure signal, the flowsignal, or both; a monitoring module that monitors an output of thedetection module to determine whether to request that the control layertake control of the pressure generating system; and a control modulethat controls the operation of the pressure generating system responsiveto the control layer being granted control thereof.
 12. The system ofclaim 10, wherein the set of prioritized control layers include: (a)flow limit control layer that monitors the flow signal to determinewhether the pressure generating system is exhibiting a large leakindicative of the patient circuit not being connected to an airway of apatient, and causes the pressure generating system to lower the IPAP,the EPAP, or both responsive to detection of the large leak andmaintains the pressure generating system at the lower pressure; (b)snore control layer that monitors the flow signal, the pressure signal,or both for snoring, and causes the pressure generating system toincrease the IPAP, the EPAP, or both responsive to detection of snore;(c) a big leak control layer that monitors the flow signal to determinewhether the pressure generating system is exhibiting a leak that is lessthan the large leak but great enough to cause the pressure supportsystem to not operate reliably, and causes the pressure generatingsystem to lower the IPAP, the EPAP, or both responsive to detection ofthe large leak for predetermined period of time; (d) an apnea/hypopneacontrol layer that monitors the flow signal, the pressure signal, orboth to determine whether the patient is experiencing an apnea, ahypopnea, or both, and causes the pressure generating system to adjustthe IPAP, the EPAP, or both responsive to detection of apnea, hypopnea,or both; (e) a variable breathing control layer that monitors the flowsignal to determine whether the patient is experiencing erraticbreathing, and causes the pressure generating system to adjust the IPAP,the EPAP, or both responsive to detection of erratic breathing; and (f)an auto-titration control layer that controls the IPAP, the EPAP, orboth responsive to proactively search for a pressure that optimizes thepressure provided to the patient to treat disordered breathing.
 13. Thesystem of claim 10, further comprising a manual input for controllingthe operation of the pressure support system, and wherein the set ofprioritized control layers include at least one first control layer thatis initiated based on the manual input and at least one second controllayer that is initiated based on the pressure signal, the flow signal orboth, wherein the at least one first control layer has a higher prioritythan the at least one second control layer.
 14. The system of claim 13,wherein the first control layer is a ramp control layer that causes thepressure generating system to gradually increase the IPAP, the EPAP, orboth from a relatively low level to a target level responsive to receiptof a ramp activation signal as the manual input.